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102 hours in pursuit of Marathon suspects

For five indelible days, the unthinkable became routine in Boston. And no one felt that more than the police and agents mounting the largest manhunt in regional history and parsing its most complex crime scene. It took a cast of thousands — also courage, sacrifice, teamwork, and luck — to crack the case. But they did it.

. . .

A quiet afternoon in his garden beckoned as Governor Deval Patrick left the finish line of the Boston Marathon. After crowning the men’s and women’s winners with wreaths of laurel, he headed home to Milton around 1 p.m., looking forward to working in the dirt.

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Boston Police Commissioner Ed Davis stayed at the race until 1:30. He was watching runners cross the finish line, but also surveying security measures at America’s oldest and most famous road race, including teams of dogs and more than 1,000 uniformed officers and soldiers.

“Be vigilant,” Davis told them in parting as he headed home to Hyde Park, to join a conference call on gun control convened by Vice President Joe Biden.

It seemed like a perfect day. The Red Sox won at Fenway. A bright sun shone. A glorious holiday, when nothing could go wrong.

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Hundreds of thousands of spectators lined the
26.2-mile course from Hopkinton to the Back Bay, many holding signs and offering cups of water to runners. In the city’s favorite rite of spring, they cheered on friends, loved ones, and strangers trailing far behind the clusters of elite athletes.

Few noticed the two young men, one in a black baseball cap, the other in a white hat turned backward, as they rounded the corner from Gloucester Street onto Boylston Street at 2:38 p.m. Each carried a bulky backpack. They strode toward the finish line where just a short time earlier the governor had hugged race organizers.

Along the way, they put down their bags, paused several minutes, and left the scene. Four days later, one of the men would be standing on a street in East Watertown, firing a gun at police, shouting, “You want more? I give you more!”

MONDAY, APRIL 15, 2:49 P.M.

The explosions

The Marathon clock, showing the time since the start of the race, flashed 4:09:43. Some 5,700 runners were still on the course. The time was 2:49 p.m. For one final instant, everything was normal.

Then, two explosions ripped through the sidelines of the race, 12 seconds and 214 yards apart, setting in motion a series of extraordinary, unthinkable, indelible events and triggering the largest manhunt in New England history. Thousands of law enforcement officers from dozens of agencies would respond before the end came four days later in a Watertown backyard, after a gunfight in the dead of night on a residential street and an unprecedented lockdown of an entire metropolitan area. Five people would die, 267 more would be injured. The city of Boston would be forever changed.

AARON TANG FOR THE BOSTON GLOBE

Boylston Street, which had been the site of Marathon revelry, was transformed into a crime scene within seconds after two bombs tore through the crowd around 2:50 p.m.

The army of law enforcement that converged at the site of the bombings — a 15-block swath in one of Boston’s most elegant neighborhoods, now the city’s largest-ever crime scene — faced forensic and tactical challenges new to almost all of them, or at least beyond their prior experience. It was a puzzle of unprecedented difficulty that began, literally, in thousands of broken, scattered pieces.

In more than 100 interviews with police, government officials, residents, and tourists who witnessed the week’s events, Globe reporters sought to reconstruct the actions of law enforcement agents between the April 15 bombing that killed three people on Boylston Street and the capture of Dzhokhar Tsarnaev seven miles away in Watertown on April 19 after his brother Tamerlan was killed, the conclusion of an epic 102-hour manhunt that left one police officer dead and another badly injured. The newspaper inquiry turned up scores of new details, fleshed out the hard questions law enforcement confronted along the way, and the harder decisions it was forced to make. It also helped put many previously reported elements in proper order and context. A maelstrom of information — some factual, some speculative, some flat out wrong — followed in the wake of the calamity, as it always does in times of tumult. This story aims to assemble what is known, and remains to be seen, in one consecutive account.

Minutes after the blasts, Davis, the police commissioner, got a call from Daniel P. Linskey, his superintendent in chief.

“I’m not sure what we got, boss,” Linskey told him, sirens wailing in the background, “but I think it’s bad. I’m hearing multiple amputations.”

With that, Davis said, he knew it was terrorism: “I started to operate on the premise it was an attack.”

Boston Police Superintendent William Evans was relaxing in a sauna at a South Boston health club, recovering from running the Marathon, when he got the call about the bombs. He ran home, donned his uniform, and commandeered a utility truck to get to work.

The governor was in his car when his daughter Katherine called him from the Back Bay.

David L. Ryan/Globe Staff

Within 20 minutes of the blasts, Boylston Street had been cleared and the wounded had been evacuated.

“Dad, what happened?” she asked him. “I heard two big booms and people are running.”

Boston Fire Lieutenant Frederick Lorenz was walking back to the race route from Shaw’s Supermarket when he heard the explosions. When he saw crowds of people running from the scene, he set his container of hot soup on a wall and started running toward the blasts.

Told there might be an unexploded third bomb under the grandstand, District Fire Chief Dennis Keeley ordered two firefighters to get down on the ground and train their hoses on the spot, covering for the bomb squad in case another fiery blast came as they crawled in to find it.

Other firefighters knelt on the ground nearby with bleeding victims, grabbing belts from bystanders and cinching them tight around nearly severed limbs. “The ones who weren’t making the noise were the ones who needed help” the most, said Lorenz.

Many of the Marathon runners themselves stopped running and started helping victims. Natalie Stavas, a pediatric resident who had been running the race with her father, immediately came to the aid of a young woman bleeding profusely from a blown-open thigh.

“As soon as I saw the wound,” she said, “I began screaming that this woman needed to be sent to a hospital.” When paramedics took the injured woman away, Stavas ran about 30 feet down Boylston until she found another young woman to treat.

No one could know it then, but the week that began in a rush of chaos and horror would end Friday night as SWAT team members with submachine guns inched forward toward the blood-stained, tarp-covered boat where the younger Tsarnaev lay hiding. Millions of people watched spellbound, wondering how the saga would end. When it was over, with Tsarnaev in custody, they rushed, cheering, into the streets.

But the ultimate success of the investigators’ mission — the capture of the suspects, one alive and one dead — depended in equal parts on luck, the heroics of police and civilians, and the startling incompetence of the Tsarnaev brothers themselves. They may have figured out how to kill, but engineering a getaway seemed beyond them.

The siblings, in many ways, made it easy for investigators. Instead of fleeing the city in the chaotic aftermath, they stuck around, seemingly unconcerned that thousands of investigators were looking for them. It took three days for the FBI to cull and then release the crucial images of the suspects from thousands of hours of video. An hour before, Dzhokhar was still at his dorm at UMass Dartmouth.

David L. Ryan/Globe Staff

Investigators mapped a grid and marked pieces of evidence at the site of the first blast.

Yet even with the seeming ineptitude of the suspected bombers, law enforcement officials didn’t know exactly who or where they were until Thursday night, after the Tsarnaevs allegedly began shooting at them in a quiet East Watertown neighborhood.

Acts of bravery were woven into the week’s fabric, from the carjacking victim who escaped from the terrorists when they stopped for gas to the Watertown police officer who faced off with Tamerlan Tsarnaev in a gunfight in the dead of night on Laurel Street, shooting at him from 12 feet away and then tackling him to the ground.

There were moments, too, that might have ended tragically but for a bolt of luck that struck when it was needed. When Watertown homeowner David Henneberry stood on a ladder and peered into his boat, the Slip Away II, noticing blood pooled inside, it was only fate or chance that the injured young man hidden there had no gun to shoot him.

From beginning to end, law enforcement efforts were hampered by confusion and misinformation, what some call the “fog of war” that envelops every emergency operation, especially one this large. Authorities first thought terrorists had also struck the JFK library in Dorchester. They battled rampant Internet rumors that labeled innocent people as suspects, and false reports in the media of an arrest. At one point on Friday, federal agents feared the younger bombing suspect had eluded their dragnet and fled to New Bedford, based on a mistaken report that his cellphone had been detected there.

In the 102 hours that passed between the bombings and the capture, law enforcement leaders faced difficult choices, about whether to release the photos of the subjects to the public, and when to lift the shelter-in-place order that transformed the city overnight into a ghost town.

And the decision-making started even before the smoke had cleared.

MONDAY, 2:55 P.M.

‘We’ve got amputations’

Police Commissioner Davis had just hung up the phone on his conference call with the White House when it rang again. It was his superintendent in chief, Dan Linskey. Davis could hear sirens in the background.

“We got two explosions,” Linskey said. “I don’t know if they’re electrical.”

Linskey, a fit officer with a shaved head, had been walking the Marathon route when the bombs exploded. As he headed back toward Kenmore Square, striding down busy Beacon Street from Audubon Circle, he heard another officer, detective sergeant Danny Keeler, screaming for help on his police radio.

Linskey could barely understand what Keeler was saying about two explosions at the finish line. But he heard the urgency in his voice and broke into a run on Beacon Street. In Kenmore Square, he leaped into another officer’s car and they barreled toward the crime scene, siren screaming, going the wrong way down the one-way street.

From the car, Linskey called his boss, who immediately figured out what was happening. As soon as Davis heard the word “amputations,” he said, he began to treat the episode as terrorism.

“A power station explosion wouldn’t cause that type of injury. That’s classic low-placement IED explosion,” Davis said.

Within five minutes, Davis said, as he tore down Turtle Pond Parkway back toward the heart of the city, he called Richard DesLauriers, agent in charge of the Boston office of the FBI.

“Rick, look, I don’t know what I’ve got,” Davis recalls telling the FBI chief, “but I have multiple explosions . . . I need to roll whatever SWAT teams you have available to Copley Square.”

“I’ll see you there,” DesLauriers told him. “I’ll get everything I can to you as quickly as I can.” It was the beginning of days of intense cooperation between local and federal agencies that sometimes turned fractious, but ended with state and city officials praising the FBI’s handling of the case.

Arriving at the scene and leaping from his vehicle, Linskey smelled gunpowder in the air. And he, too, knew then what they were dealing with: bombs.

The street was bloody chaos, a battleground. Bodies and body parts were strewn on the pavement, and victims covered with towels. We ran out of ambulances, Linskey remembers someone telling him. The street was littered with shrapnel, nails and ball bearings, and belongings dropped by fleeing spectators: cellphones and backpacks; duffel bags and handmade signs with runners’ names.

Arriving on Boylston, Davis was struck by the contrast, the dramatic transformation: “No one but uniforms at this place where I had just been with 100,000 people.”

He walked down the street to examine the blast sites. It was then, Davis says, that a driving sense of urgency kicked in, a feeling of danger and momentousness that added an intensity to every step that followed.

“I really believed . . . we had a terrorist on the run that we needed to capture as soon as possible,” said Davis

Fearing a third bomb, he moved quickly to lock down the area, pushing people and police back away from the bomb sites while specialized teams swept the sea of debris. It was not the only incorrect assumption of the day: For a time, Davis and others erroneously suggested a mysterious fire at the John F. Kennedy Presidential Library and Museum that broke out about the same time as the bombings was terror-related.

State and city police and emergency management leaders conferred and settled on the Westin Copley Place hotel for their command post; the Fairmont Copley Plaza had a function already set up in its ballroom. By 4 p.m., a dozen people gathered there around a single table including Davis, Colonel Tim Alben of the State Police, the FBI’s DesLauriers, US Attorney Carmen Ortiz, and District Attorney Daniel F. Conley. An hour later, the crowd had swelled to 100. And the officers kept coming: city, state, transit police; FBI; ATF. Ultimately, more than 20 law enforcement agencies would take part in the manhunt with more than 1,000 investigators working out of the third and fourth floors of the Westin alone.

They had to act quickly. Officials had initially activated the alternate Marathon route, but then ended the race to stem the flow of more than 5,000 remaining runners toward the bloody finish line. Still wary of further attacks, they began dispatching police to other sites around the city — Faneuil Hall, hospitals, train stations. As fear and confusion swept Boston, they knew they would field calls about suspicious people, packages, and bags.

Within minutes of establishing the command post at the Westin, Davis said, he issued the first order to collect video. Investigators, led by Boston Police Sergeant Detective Bill Perkins, began going after surveillance cameras from surrounding businesses. Senior officials began to gather.

One of them was Conley, the Suffolk DA, who had earlier been watching the race at Abe and Louie’s restaurant. He almost became physically ill when he realized how close he and his wife had been to the bombers — less than 50 feet.

“I had to pull myself together,” said Conley. “I could have rubbed elbows with the guy. Did I pass him? If my friend hadn’t said, ‘Are you ready to leave’ . . . I felt fortunate, and at the same time I felt sick for the loss of life.”

Mayor Thomas M. Menino, who just days before had undergone surgery for a broken leg, checked himself out of Brigham and Women’s Hospital, where many victims were taken, and made his way downtown to the command post, too. The mayor had already missed one of his favorite springtime duties — crowning one of the marathon winners — but he wasn’t going to stay sidelined during an attack on the city he has led for 20 years.

“The doctors said, ‘You shouldn’t be going,’ ” Menino recalled. “I said, ‘I don’t care what you say, doc. I’m going.”

Police first briefed the media soon after 5 p.m. An hour later, shortly after 6, President Obama addressed the nation, promising the “full resources” of the federal government.

“Today is a holiday in Massachusetts; Patriots Day,” he said. “It’s a day that celebrates the free and fiercely independent spirit that this great American city of Boston has reflected from the earliest days of our nation, and it’s a day that draws the world to Boston’s streets in a spirit of friendly competition. Boston is a tough and resilient town, and so are its people . . . The American people will say a prayer for Boston tonight.”

While Americans prayed, law enforcement analysts at FBI headquarters in Boston’s Center Plaza got to work reviewing the torrent of videotapes of Boylston Street that were streaming in from businesses and ordinary spectators alike. Eventually, more than 100 analysts were painstakingly reviewing thousands of hours of surveillance and amateur videotape for anything suspicious — anyone resembling a bomber about to strike.

By 8 or 9 p.m. Monday, federal agents were taking the reins of the investigation. It was a seamless, logical transition, said Patrick, and “there wasn’t any fussing about it.”

“One of the first things we did Monday night was get all these agency heads in a room and say, OK, who’s in charge,” said Patrick. “We need[ed] one agency in charge … The logical lead was the FBI and the Joint Terrorism Task Force.”

The governor reflected later on his first reaction, when his daughter called him in his car to ask about the bombings. Instinctively, Patrick had offered reassurance, telling her, “I’m sure it’s OK.”

“Because you want to believe it, right?” he said. “That it’s OK.”

But Patrick knew now that things were far from OK.

As the night wore on, and investigators descended on hospitals to question witnesses, more and more average Bostonians joined the expanding quest for the bombers.

Investigators frantically searched for video. Kiva Kuan Liu, a Boston University graduate student, had been filming footage for a documentary near where the first bomb exploded. She was summoned to Tufts Medical Center around 10 p.m. to be interviewed. In a sterile hospital meeting room, Liu said, four investigators grilled her. When her memory failed her, they persisted.

“They were very picky about details,” said Liu, 23, who was not injured in the blast. Liu volunteered her Panasonic video camera, which she had borrowed from BU, but insisted they give her a phone number so she could get it back later.

Meanwhile, the alleged bombers had quietly disappeared into the city. As surgeons tried to save scores of people, one or perhaps both of the brothers went shopping.

A grocery receipt recovered by police suggests that, shortly after the Marathon bombings, at least one of the brothers apparently bought groceries at a Whole Foods store in Cambridge, about a half-mile from their family home on Norfolk Street. A person with knowledge of the investigation said the FBI seized video surveillance equipment from the store on Prospect Street after finding a receipt in one of the brother’s pockets after Tamerlan was killed and Dzhokhar arrested.

Little is known about Tamerlan’s movements after the bombing, though neighbors said he spent a lot of time inside his apartment taking care of his 3-year-old daughter while his wife worked. But Dzhokhar, the suspect with the white hat on backward, was an open book.

A seemingly normal college kid in many ways — the 19-year-old UMass Dartmouth sophomore played soccer and liked to smoke pot, said his friends — Dzhokhar was also an active Twitter user, the popular online social media site.

Only hours after the bombings, he took to Twitter to comment on the tragedy, seeming as removed from the shattering events as any other college student watching from a distance.

“Ain’t no love in the heart of the city,” he wrote. “Stay safe people.”

His last Twitter message Monday night was opaque and serious.

“There are people that know the truth but stay silent,” Dzhokhar wrote at 12:34 a.m. Tuesday, “& there are people that speak the truth but we don’t hear them cuz they’re the minority.”

He would continue to stay silent. But the truth would soon emerge.

TUESDAY, 5:51 A.M.

Daylight and devastation

As a new day dawned on Boylston Street, the light revealed a tragic transformation in the heart of one of Boston’s liveliest and best-loved neighborhoods.

Within a 15-block zone shut down by police, hundreds of investigators had overtaken the street and sidewalks. Meticulously, stepping over broken glass and bloodstains, they documented the evidence, labeling every object of interest with a spray-painted orange number beside it before bagging it and transporting it to a warehouse in the Seaport district for analysis.

Aram Boghosian for The Boston Globe

By early Tuesday, a memorial for the victims of the attacks began to form at Boylston and Berkeley streets.

With the same orange paint, they mapped a massive grid across the crime scene, dividing it into rectangular sectors: 2A directly in front of LensCrafters. 1A directly in front of the business next door. 2B and 1B closer to the street, and on and on across the pavement.

Chemists, explosive experts, and crime scene analysts — more than 30 in total from the ATF — continued combing the blast scene, where the moment of the explosions seemed frozen in time.

Spectators had left behind purses and homemade signs. Baby carriages were abandoned, and so were drinking glasses, some miraculously still half full.

“There were really thousands of pieces of evidence to look at,” one investigator said.

The blasts were so strong that some items, including the lid from a pressure cooker, were found on nearby roofs. Piecing together fragments, agents determined that the bombs were probably fashioned from pressure cookers, filled with nails and ball bearings to increase the carnage.

Overnight developments in the investigation fueled a rising tide of online rumors that would at times threaten to overwhelm police.

Authorities confirmed that late on Monday they had interviewed a Saudi national believed to be a student in Boston, and had searched an apartment building in Revere. The man, who was injured in the blast and admitted to the hospital, cooperated with the FBI and told agents he was not involved.

He would not be the only innocent person temporarily ensnared by an anxious public’s rush to judgment. Later in the week, the New York Post would splash on its front page a photo of two people later determined to have had nothing to do with the attack, under the inflammatory headline “Bag men.”

Rumors flew on Tuesday as residents’ mood remained jittery. Security fears disrupted two flights at Logan Airport. At the Cathedral of the Holy Cross in the South End, preparations were beginning for an interfaith prayer service where Obama planned to speak on Thursday.

Thousands of photographs and videos were pouring in from businesses, tourists, and residents. Investigators pored over the images, replaying them again and again and searching for people whose actions seemed out of synch with the crowd around them.

BILL GREENE/GLOBE STAFF

Officials spoke to the media at their command post, the Westin Copley Place hotel. Ultimately, 1,000 investigators would be based there.

It was a daunting task, performed with urgent energy.

“The sheer volume of stuff made it difficult to sort through,” said Alben, 53, the State Police colonel appointed last summer to oversee the state’s 2,300 troopers. But, he said, “we were very confident that there were enough cameras down there that we were going to capture something” that would lead to bombers.

As the investigators stared at their screens, people around the country were also glued to theirs. The youngest person killed by the bombers, 8-year-old Martin Richard of Dorchester, had been identified by 2 a.m. Tuesday.

Later that day, a photograph of the third-grader holding a homemade sign with the slogan “No more hurting people. Peace,” was ricocheting around the Internet, a poignant symbol of the city’s loss.

Two other families were also grieving. In Medford, the grandmother of 29-year-old Krystle Campbell struggled to come to terms with the loss of her good-natured, reliable caretaker, while the Boston University community would soon learn the third victim was Lu Lingzi, 23, a bubbly, hard-working graduate student who was ecstatic to be studying statistics in Boston.

Menino, Patrick, and Senator Elizabeth Warren visited the injured in the city’s hospitals Tuesday, while in the Ashmont section of Dorchester, young classmates of Richard filed somberly up the walkway of his house to leave flowers on the steps.

One of Richard’s alleged killers, meanwhile, was on a different sort of errand.

Around 12:30 p.m. Tuesday, Dzhokhar Tsarnaev showed up at Junior Auto Body shop on Columbia Street in Somerville, a short walk from the Tsarnaevs’ home on Norfolk Street on the Cambridge-Somerville line. He was there to pick up a white Mercedes-Benz he had left there two weeks earlier, a car he had said belonged to a girlfriend.

The car’s rear bumper wasn’t fixed yet. But Dzhokhar told the mechanic, Gilberto Junior, he would take it anyway.

“I need it now,” Dzhokhar said.

Normally the 19-year-old was relaxed and happy, chatting about soccer, smoking weed, and girls. But on the day after the bombing, Junior said, Dzhokhar appeared anxious. He was biting his nails, and his knees shook so much that Junior thought the kid had been “popping pills.”

“I wish I knew,” Junior said later, looking at his hands.

By Tuesday afternoon, Dzhokhar was back on campus at UMass Dartmouth, according to UMass Police Chief Emil R. Fioravanti. Card swipe records, videotape, and interviews with students show the 19-year-old sophomore spent time on campus between Tuesday and Thursday, said the chief.

At 9:05 p.m. on Tuesday, the college sophomore used his swipe card to enter the Fitness Center, a small room packed with free weights, weight machines, running machines, and flat-screen TVs. Outside the fitness center, Dzhokhar showed up for 5 or 10 seconds on a security camera.

He also posted more Twitter messages Tuesday, including one about a photo of a bombing victim. Internet rumors swirled about the injured woman in the picture, claiming her boyfriend had planned to propose, but found her dead.

John Tlumacki/Globe Staff

Investigators combed through debris at the site of the second blast. A shredded backpack, a piece of the pressure-cooker bomb, and its electronic parts were found.

“Fake story,” Dzhokhar tweeted.

That night he also posted lyrics from a song by the rapper Eminem: “Nowadays everybody wanna talk like they got something to say but nothing comes out when they move their lips; just a bunch of gibberish.”

The suspect appeared to be resuming his normal life, unconcerned that he could be caught, working out at the gym instead of hiding or running.

“I’m a stress-free kind of guy,” he tweeted after midnight.

But back in Boston, investigators combing through hours of footage on Tuesday had begun to see a pattern among the various videos, their attention drawn especially to two young men walking east along Boylston Street in the 12 minutes before the explosions.

Neither seemed particularly worried about hiding their appearance — the man in the white hat even turned his cap around backward, giving the cameras a full shot of his profile. Suspect number 2, as he became known, talks nonchalantly on his cellphone, then scarcely reacts at all when the first bomb goes off.

When Alben, of the State Police, saw the results of the analysts’ work on Wednesday morning, he couldn’t believe it: they had captured an image of the young man in a white hat dropping a backpack outside the Forum restaurant and then walking away.

“There was a eureka moment . . . It was right there for you to see,” said the colonel. “It was quite clear to me we had a breakthrough in the case.”

WEDNESDAY

Pinning down identities

They had faces. Now they needed names.

Even after authorities isolated the images of the two suspected bombers, they weren’t able to pinpoint the suspects’ identities — an essential puzzle piece that was still missing Wednesday.

The FBI has poured millions of dollars into facial recognition technology over the years so it can quickly cross-check an image against millions of other pictures in government databases.

In this case, both brothers were already in existing government databases, including the Massachusetts Registry of Motor Vehicles and federal immigration records. They were legal immigrants from the former Soviet republic of Kyrgyzstan, in theory allowing the FBI to find their names.

But it’s not clear which databases the FBI checked. And it may not have mattered. The pictures, taken from surveillance cameras above street level, were likely far too grainy when zoomed in on the brothers’ faces. And the older brother was wearing sunglasses, making their task even harder.

In addition, unlike in a traditional mug shot, the camera wasn’t looking at their faces head-on.

Investigators worked feverishly Wednesday trying to identify the men, searching other photos and video, trying to find high-definition images.

“We still needed more clarity,” said Alben, of the State Police. “As good as the videos were, we needed more clarity.”

They continued hunting down new images. David Sapers, owner of the Boylston Street candy store Sugar Heaven, said he had called the FBI hot line on Tuesday to inform authorities that he had video. Wednesday morning, Homeland Security officers showed up and spent more than three hours reviewing the footage frame by frame with the store’s manager, said Sapers.

Colonel Alben, the State Police chief, briefed Patrick on Wednesday about the key piece of video showing Dzhokhar abandoning his backpack. Alben described the clip and showed the governor photographs culled from the footage, information that Patrick called “chilling.”

“We have a break,” Patrick remembers him saying. “We think we have a face.”

If further proof was needed that the city was on edge, the anxiety soon spilled over into view. By 1 p.m., news reports began surfacing that a suspect had been not only identified but arrested, and was headed to the federal courthouse. Hundreds of reporters and photographers descended on the Moakley courthouse in South Boston. The Coast Guard and Boston Police Department Harbor Patrol patrolled nearby waters.

Relying on information from a source familiar with the investigation, the Globe posted a report online for a short time Wednesday that a suspect was in custody and en route to federal court. The FBI later issued a statement denying the arrest.

Inside, courthouse staff members, including some from the US attorney’s office, flocked to the emergency magistrate’s courtroom, anticipating a hearing, based on breaking news reports from the Associated Press, CNN, and others.

Court staff began discussing whether they should move the hearing to a larger courtroom with more seating, and whether to provide a live, closed-circuit feed to yet another courtroom, in anticipation of an overflow crowd. Watching the drama unfold on live television, the US attorney’s office denied the reports of an arrest, and called court officials to tell them there would be no hearing.

But by then, word of the arrest turned into an avalanche of chaos, silenced only by the “code red” that was broadcast over the intercom at 3:01 p.m. The building’s management company had received a bomb threat, and the US Marshals Service ordered an evacuation. Outside, judges mixed with members of the public who had come to catch a glimpse of the commotion.

It was a code none of the lawyers had heard before, and none of them knew what it meant. But they could tell from the seriousness in the announcer’s voice that something was up.

“This was different; this was louder,” Boston-based lawyer Jonathan Shapiro thought as he descended seven flights of stairs to exit the building. Outside, seeing the patrol boats with machine guns and the crowd in the streets, he was struck by the oddness of the moment.

“I’ve never seen anything like it,” he said.

After a sweep by federal agents, the area was cleared at 4:41 p.m., and courthouse employees were allowed back in.

As the hours passed and the suspects’ identities remained elusive, pressure was mounting inside the command center, now at FBI headquarters across from City Hall.

A key question, with game-changing consequences, was bearing down on the men in charge: to release the photos to the public or to hold them close?

The wrong move could be deadly. But with the bombers still at large, they had to decide soon.

THURSDAY, 11 A.M.

Beginning to heal

Under soaring stone arches, in light filtered by stained glass, hundreds of residents sought comfort Thursday morning inside the Cathedral of the Holy Cross. Outside, bomb-sniffing dogs patrolled the streets, but inside there were words of encouragement and healing.

“That’s what you’ve taught us, Boston,” President Obama said at the interfaith service. “To persevere. To not grow weary. To not get faint. Even when it hurts. Even when our heart aches. We summon the strength . . . and we carry on. We finish the race.”

SUZANNE KREITER/GLOBE STAFF

As investigators pored over a mountain of evidence, areas of Boston, such as City Hall Plaza, remained on high alert.

Investigators, too, had a race to finish. And it was almost time for the final sprint.

As the president traveled from the packed cathedral to the Boston hospitals where he sat with victims, Homeland Security Secretary Janet Napolitano became the first official to say publicly that there may be two suspects. She told the House Homeland Security Committee “there is some video that raised the question” of two men the FBI would like to interview in connection with the bombings. She did not share any details about the video.

In Boston, meanwhile, law enforcement leaders were debating their next step. State Police Chief Alben said FBI, Boston Police, and State Police leaders wrangled over whether to release the photos of the suspects. Everyone weighed in, but the final call was the feds’.

The potential payoff — a quick ID from a tipster — was huge, given that the bombers were still out there on the loose and could be plotting another attack. But the risks weighed heavily on them.

“There is a huge conundrum here that if you release the photos, if they haven’t fled the Boston area they are going to flee,” Alben said. “You would always prefer to identify them yourself. You always want to apprehend someone when you have control of the situation, not when someone has been tipped you’re coming through the door.”

Alben said investigators were also concerned that the media was publicizing video and photographs from the Marathon, wrongly identifying various people as suspects.

Still, Alben was worried about the decision to go public. “In my mind it was clear that even if someone [from the public] couldn’t identify them, [the suspects] would know we had them,” he said. “I was concerned they would flee the area or would go so far underground you never would be able to find them.”

At 2:50 pm on Thursday, the FBI announced on its website that it would hold a press briefing at 5 p.m.

THURSDAY, 4:02 P.M.

Anonymous no more

Sixty miles away at UMass Dartmouth, Dzhokhar Tsarnaev used his swipe card to enter his dorm at 4:02 p.m. He had one more hour of anonymity remaining.

It would be the last time he would use his card on campus.

The 19-year-old had already made what would be his last posting on Twitter, the previous day. He retweeted a message from a Saudi scholar, Mufti Ismail Menk: “Attitude can take away your beauty no matter how good looking you are, or it could enhance your beauty, making you adorable.”

After that, silence.

BOB LEONARD

The FBI released images of the suspects, later identified as Dzhokhar and Tamerlan Tsarnaev. A night of lethal violence was to follow.

Dzhokhar had enjoyed three days of freedom since the bombings. But he and his brother Tamerlan had failed to prepare for what was coming, as if they had imagined investigators would never track them down.

Dzhokhar’s life as a “stress free kind of guy” who sold pot on the side to raise pocket money was about to end. By the time he arrived back in Cambridge sometime that night, his face would be known to millions, and tips about his identity would be pouring in to federal agents.

“For more than 100 years, the FBI has relied on the public to be its eyes and ears,” DesLauriers, the FBI agent in charge, told the nation at the 5 p.m. press conference. “With the media’s help, in an instant, these images will be delivered directly into the hands of millions around the world. We know the public will play a critical role in identifying and locating them.

“Somebody out there knows these individuals as friends, neighbors, co-workers, or family members of the suspects. Though it may be difficult, the nation is counting on those with information to come forward.

“No bit of information, no matter how small or seemingly inconsequential, is too small. Each piece moves us forward towards justice,” he said.

DesLauriers cautioned the public to be careful, and to consider the two men armed and “extremely dangerous.”

“No one should approach them,” he said. “No one should attempt to apprehend them except law enforcement.”

Back at UMass Dartmouth, Pamala Rolon returned from class and turned on the TV news Thursday night, where pictures of the bombing suspects flashed across the screen. One of them looked faintly like a guy she knew on campus.

“We made a joke, like, that could be Dzhokhar,” she said. “But then we thought it just couldn’t be him. Dzhokhar? Never.”

She wasn’t the only one to see Dzhokhar’s picture on TV and make a joke. One of his Twitter followers even sent him a copy of his image from the FBI pictures, writing, “Is this you? I didn’t know you went to the marathon!!!!”

The brothers’ sudden notoriety may have inspired their belated and desperate plan to escape to New York, where authorities now believe the brothers had drawn a bull’s-eye on their next target: Times Square, the fabled “Crossroads of the World.’’

But the brothers hadn’t set aside money or even a getaway car for the journey — they had to figure that the green Honda Civic Dzhokhar drove would soon be known to police. They were also seriously short on weapons — police recovered only one handgun and a BB gun that they could trace to the pair.

Their apparent hope of adding to that paltry firepower set them on a deadly collision course with Sean A. Collier, a genial police officer at the Massachusetts Institute of Technology.

Collier grew up in Wilmington with five siblings who say he was born to be a cop — and he was to begin a new job this summer with the Somerville Police Department.

“He came to see me a couple of months ago and he said, ‘Chief, I have a chance to get on the Somerville Police,” said MIT Police Chief John DiFava. “I said, ‘Sean, you owe me nothing. You’ve done a fine job for me. I would never stand in the way of someone trying to do better for themselves.’’

As the Tsarnaevs headed toward the MIT campus, Collier was on duty near Kendall Square in Cambridge and DiFava pulled his car next to Collier’s cruiser.

In the days after the Marathon bombings, the MIT chief had ordered additional security for the campus. And Collier, who began work on the MIT force in early 2012, was nearing the completion of his 3 to 11:15 p.m. shift.

About 9:30 p.m., Collier was on routine patrol. He was parked by the corner of Vassar and Main streets. It was a spot where motorists would sometimes take a chance, making an illegal shortcut through campus to avoid a red light.

“We ask patrols to sit there,’’ DiFava explained. It prevents the forbidden cut-throughs and it provides a high-profile presence for the MIT community.

What are you doing? the chief asked his young officer.

“Just making sure everybody behaves,’’ Collier told him.

The two men chatted easily for several minutes. And then DiFava pulled away.

The chief was home for perhaps a half hour when his phone rang. “It was the deputy chief,’’ DiFava said. “He said Sean Collier has been shot.’’

In the fog of what would become a deadly and dangerous night, there were initial reports that Collier had been responding to a disturbance. That turned out to be wrong.

What actually happened was more cold-blooded, authorities said. Police officials have called it an assassination, an execution.

Authorities say video from a surveillance camera shows the suspects approaching Collier’s car from the rear as he sat in his cruiser. Collier was shot five times, including twice in the head, officials said.

“He didn’t stand a chance,’’ DiFava said.

The two bombing suspects allegedly tried to steal Collier’s weapon, but they couldn’t unlock it.

“The retention holster does its job well, so perhaps they didn’t get the gun because of that holster,’’ the MIT chief said. “Maybe that’s what thwarted them from getting the gun, because the gun was not removed from the holster.’’

Authorities say it is not clear why the Tsarnaevs were at MIT or why they targeted Collier.

“We have all kinds of unanswered questions,’’ DiFava said.

Last week, the Somerville Police Department asked the city’s Board of Aldermen to honor Collier, appointing him posthumously to the city’s police force.

As tributes flooded in about Collier, including those from Biden, DiFava could scarcely utter words about his fallen officer.

“From the time I saw him to the time he was dead, it was probably about an hour,’’ DiFava said.

If the details of the attack were unspeakable, they proved to be useful weapons for the alleged terrorists.

“I just killed a policeman in Cambridge,’’ Tamerlan Tsarnaev told the man he would meet next as he waved a silver handgun at him.

THURSDAY, 11 P.M.

Carjacked

It was a cool spring night, and a 26-year-old entrepreneur from Cambridge had picked the perfect way to unwind: take his new $50,000 Mercedes-Benz out for a spin.

As he drove his spacious 2013 black SUV toward Boston, the man — a Chinese immigrant who asked to be identified only by his American nickname, Danny — noticed a swarm of police cars with flashing blue lights near the MIT campus.

After a short drive across the river, Danny, who earned a master’s degree from Northeastern University last year and is trained as an engineer, pulled his car to the curb on Brighton Avenue to answer a text message.

Suddenly, an old sedan swerved behind him and slammed to a stop. A thin young man in dark clothes got out, approached the passenger window, and rapped on the glass. Danny lowered the window.

The man reached an arm through, unlocked the door, climbed in, and pointed a handgun.

Aram Boghosian for The Boston Globe

MIT Police Officer Sean Collier was shot to death late Thursday near the school in Cambridge. The alleged bombers were suspected.

He demanded cash, and made it all too clear this wasn’t a joke.

“You know about the Boston Marathon bombing? I did that,” Tamerlan said. “Don’t be stupid.’’

What followed was a harrowing 75 minutes during which 26-year-old Tamerlan Tsarnaev — soon joined in the car by his brother Dzhokhar — forced Danny on a circuitous journey through Brighton, Watertown, and back to Cambridge.

The brothers wanted more cash than the $45 that Danny had and they talked about driving to New York. Otherwise, their intentions were unclear. The odyssey took on a surreal tone. The brothers marveled at the features of the Mercedes-Benz, popped in a CD of what sounded like Middle Eastern religious music, and talked about credit limits for students.

Through it all, Danny feared they would kill him if he made one misstep. He frantically thought about how to escape. And he prayed.

They lapped Brighton and crossed the Charles River into Watertown, following Arsenal Street. Opening Danny’s wallet, Tamerlan asked for his ATM code.

Then he instructed Danny to pull over on a side street in East Watertown. A Honda Civic sedan that Danny had noticed following him stopped behind the Mercedes. A younger man approached – the shaggy-haired “Suspect No. 2’’ in the photos and videos released by authorities just hours earlier. Tamerlan got out, ordered Danny into the front passenger seat, and warned him that if he tried anything he would shoot him. For several minutes, the brothers moved heavy objects from the smaller car into Danny’s SUV; Danny figured it was luggage.

Tamerlan took the wheel and his brother got in the back seat, leaving the Honda behind. Danny looked at the younger man and recognized Dzhokhar from the FBI images.

“I saw his face because he had a clear picture,’’ Danny said. “I was 100 percent sure that he was really similar to that guy.’’

At one point, Danny asked Tamerlan, “Are you going to hurt me?’’

“I’m not going to hurt you,’’ he replied. “We’re just going to drop you off. . . . Probably you’ll have to walk 4 or 5 miles to find anybody, and if you are lucky, somebody will pick you up.’’

The Mercedes stopped in front of a Bank of America ATM in Watertown Center. The younger brother got out and withdrew an estimated $800. Danny debated bolting from the Mercedes. He implored the older man to let him unbuckle his seatbelt and put on a jacket in the back seat because he was cold.

“This is a good time to run,’’ he thought. “But it was just so tough.’’

As religious music blared over the stereo, Danny received a text message from his roommate just before midnight. Tamerlan demanded to know who was texting and what was she saying.

He grabbed the phone and used an English-to-Chinese app to text a response that Danny was sick and sleeping at a friend’s house. The message was riddled with grammatical errors. His roommate, alarmed, texted again.

Seconds later, at 12:03 a.m., the roommate’s boyfriend called. Tamerlan was furious.

He took out the silver gun and pointed it at Danny.

“Answer it,” ordered Tamerlan. “If you say a single word in Chinese, I will kill you right now.”

Danny pleaded and prayed. He asked Tamerlan precisely what to tell his friend so that he didn’t make a mistake.

“Where are you?” his roommate asked in Mandarin.

“I’m gonna sleep in my friend’s place tonight,” Danny replied in English as he stared at the gun. “I’m sorry I have to go.”

Tamerlan congratulated him on a job well done.

Tamerlan drove down Soldiers Field Road, passed Harvard Business School, turned left on a bridge in front of the Doubletree Hotel, and then another left into the Shell Station on Memorial Drive to buy gas.

“Maybe I have a chance,” Danny thought. “It’s my last chance to get out.”

Tamerlan pulled up the car with the gas pumps on the right, next to Danny. Dzhokhar got out with Danny’s credit card to pay for the gas.

It was around 12:15, an hour and a half since he was abducted. Danny realized it was a critical moment: The doors were unlocked. Dzhokhar was in the store. The gun was tucked in the driver’s side door. And Tamerlan was distracted, struggling with the GPS device he had brought with him on the carjacking.

“I was thinking I must do two things: unfasten my seatbelt and open the door and jump out as quick as I can. If I didn’t make it, he would kill me right out, he would kill me right away,” Danny said.

Tamerlan tried to grab his jacket. But Danny slammed the door, leaving his phone in the vehicle. He ran behind the SUV at an angle to avoid potential gunshots. “F---!” Tamerlan screamed as Danny darted across River Street to a Mobil Station that had the lights on.

In the instant that Danny sprinted to freedom and toward a 911 telephone call, the brothers were now fully exposed.

They were driving a stolen vehicle.

They were the most wanted men in New England.

And as they roared out of Shell’s parking lot and onto Memorial Drive, they pointed the black Mercedes toward Watertown.

FRIDAY, 12:44 A.M.

Shoot-out in Watertown

Watertown Police Officer Joseph Reynolds’s midnight to 8 a.m. shift had barely begun when police frequencies erupted with news of abduction and mayhem next door in Cambridge.

“Wanted for carjacking that occurred in Cambridge, possibly related to the Cambridge incident,” said the State Police dispatcher, referring to the shooting at MIT. “Middle Eastern males, the victim stated they both had handguns … during the conversation, the victim gathered they were possibly heading to New York City.”

Mario Tama/Getty Images

A report of a carjacking by the suspects led to a gunfight early Friday in Watertown. Transit Police Officer Richard Donohue Jr. was wounded in the shootout.

In fact, the black SUV stolen by the Tsarnaev brothers was nowhere near New York City. The global positioning system on the vehicle, which emitted tiny traceable electronic signals, showed that the Mercedes was less than 5 miles from their apartment – and heading Reynolds’s way.

At 12:42 a.m., Watertown’s dispatcher warned: “Okay, the vehicle is now in Watertown, units, in the area of 89 Dexter,” part of a slumbering neighborhood of tidy houses and duplexes whose residents proudly decorate their homes with flower boxes. A lot of Watertown was like that — a small town where many of the police officers never fired their guns outside of the practice range.

“I’m right behind that vehicle,” Reynolds replied.

The patrol supervisor, Sergeant John MacLellan, advised caution. “Don’t stop the car until I get there,’’ he told Reynolds, according to Watertown Police Captain Raymond Dupuis. “Wait for help to come.’’

But there was no time. The SUV took a sudden left turn from Dexter Avenue onto Laurel Street and came to an abrupt stop — right behind a second vehicle, the green Honda Civic that Dzhokhar Tsarnaev had retrieved moments earlier. As Reynolds watched, one brother emerged from each car.

“The suspects get out and start shooting at Joe Reynolds,’’ said Dupuis.

The next four minutes may go down as the longest in Watertown history, as the two sides fired more than 250 bullets all told and the brothers hurled bombs, filling the night air with the stench of sulfur. When the smoke cleared, one bomber was dead, a police officer had been gravely wounded, houses were pockmarked with bullet holes and shrapnel, and an entire community was traumatized.

Officers who had been guarding the crime scene at MIT sped off to join the fight as soon as the Watertown dispatcher said, “Shots fired, all units respond.” Flashing blue and white lights of dozens of cruisers lit up the night as they converged on the bridges leading from Boston to Cambridge.

“There are [expletive] bombs, they’ve got [expletive] IEDs,” shouted one FBI agent as he ran through East Watertown toward the shoot-out. “Everyone get your [expletive] phones off. No phones. They’ve got IEDs,” referring to improvised explosive devices.

When the shooting started, Reynolds jammed his cruiser into reverse, trying to gain distance between him and his attackers. Moments later, help arrived as shift supervisor MacLellan rounded a corner in his black-and-white Ford Expedition and immediately had a bullet graze his front windshield.

The patrol supervisor jumped out of his car and took cover behind a tree at the corner of Dexter Avenue and Laurel Street. The Tsarnaevs, using the SUV now as a shield, continued to fire.

By now, Laurel Street slumbered no more, though at least one resident thought the growing commotion was caused by children playing with firecrackers.

“Get the hell out of here and go to your own neighborhood,” Peter Kehayias yelled out the window of his two-family house.

“Get inside and shut your window,” an officer commanded loudly.

Then the police officer screamed at the young men standing in the street, “Give up! There’s no way out! Give up!” as Kehayias and his wife, Loretta, watched.

Tamerlan Tsarnaev, standing in front of the black Mercedes SUV parked in the middle of the street, brazenly taunted: “You want more? I give you more.”

The doors of the SUV were wide open as Dzhokhar reloaded the pistol and handed it to Tamerlan, Kehayias said. Dzhokhar then reached inside for a duffel bag.

Loretta Kehayias, a special education teacher in Cambridge, picked up the phone and called 911: “Do you people realize I believe there is a cop out here and there are two guys? . . . They’re shooting at him.”

The reply was instant: “Yes we know, lady.” Click.

MacLellan returned to the Expedition, put it in neutral, and exploited the gentle incline of the street to push the vehicle toward the brothers, strobe lights on frenetic flash. He wanted the gunmen to think he was still in the Expedition so if the siblings fired at it, he might see them and get a clear shot.

More help arrived. Officer Miguel Colon, who joined the force with Reynolds in 2006, came around a corner and drew instant fire, one round shattering the spotlight on his cruiser.

As the Expedition rolled past her house, Lizzy Floyd crouched with her husband beneath a bedroom window on the second floor of their home, witnessing the startling bursts of gunfire and something more alarming. Floyd felt the neighborhood shake as the young men threw what appeared to be pipe bombs.

“We have them pinned down and they’re throwing explosives at us,” an officer reported, according to recordings of the scanner traffic that night.

At one point, Dzhokhar allegedly pulled out another pressure cooker bomb — like the ones they allegedly set off at the Marathon — and hurled it toward the police. The explosion created an instant bright yellow flash that turned the midnight darkness to day and knocked a framed photograph of a New Hampshire harborside off Floyd’s shelf.

One of the officers rushing to join the fight was Watertown Sergeant Jeffrey Pugliese, a 33-year veteran and a trained firearms instructor. His arrival would turn the tide.

Pugliese ran along the side of one house, through its back yard, jumped a chain-link fence and circled back, walking up a driveway to within 12 feet of the older Tsarnaev. The two men started shooting at each other.

“Sergeant Pugliese feels he hit the suspect a number of times,’’ said Dupuis, noting that Pugliese is an excellent shot. “If he says he hit him, he hit him. The suspect’s shooting and returns fire but he misses.’’

When Tamerlan ran out of ammunition, he threw his weapon at Pugliese, hitting him in the arm, Dupuis said. Tamerlan then tried to run, but Pugliese, with the aid of Reynolds and MacLellan, tackled him in the street and handcuffed him.

Dzhokhar Tsarnaev may have injured himself in one of the blasts — he had what appeared to be shrapnel cuts on his neck and ear when he was captured — but that didn’t stop him. He jumped into the stolen SUV and started driving straight toward the three officers and Tamerlan.

“He’s coming toward us!’’ MacLellan yelled in time for the other officers to roll off Tamerlan at the last minute.

The SUV ran over Tamerlan Tsarnaev with a sickening thump. Blood pooled around him. Red streaks stained the pavement where Dzhokhar had dragged his older brother under the SUV.

“He was on his belly; he was moving,” said Jean MacDonald, who was watching from her second-floor bathroom window on Laurel Street. “I saw him trying to lift up his head.”

Police said Tsarnaev dragged his brother’s body about 30 feet.

“I could see the SUV headlights go up and then down when he drove over his brother,” said Rob Mullen of Laurel Street, who watched the gunfight unfold in disbelief.

Somehow, Dzhokhar Tsarnaev maneuvered the careening SUV between the two police cars on Laurel Street and sped off at 12:48 p.m. Wounded, he didn’t get far, abandoning the vehicle about half a mile away and setting off on foot.

“Police were heroic,” said Jane Dyson of Dexter Avenue. “They stood their ground. They did not retreat an inch.”

FRIDAY, 12:51 A.M.

Race to rescue

“Gunshots. Officer down.’’

When that alert pierced the silence of the small firehouse on Orchard Street in Watertown — the city’s north station — firefighters Patrick Menton and Jimmy Caruso — both trained as emergency medical technicians — jumped into a boxy ambulance and roared toward Laurel Street.

There are few more urgent words in public safety than “officer down.’’ For Patrick Menton they buzzed now with extra electricity. His younger brother Tim is a Watertown police officer. Is the officer in trouble my brother? Menton wondered.

They sped down Orchard Street to School Street. Mount Auburn to Laurel.

“Get some rubber gloves out,’’ Caruso told Menton. “Get ready.’’

The seven Watertown officers who responded first were quickly being augmented. State Trooper Christopher Dumont, a paramedic, and Linskey, the Boston Police Department’s superintendent-in-chief, had joined the response.

Other reinforcements poured in. Just blocks from the shooting, other officers emerged from their vehicles clutching weapons and listening for instructions.

As Dzhokhar Tsarnaev escaped, officers returned to Tamerlan to check on his condition when urgent word went out. “Officer down.’’

In all the chaos, MBTA officer Richard H. Donohue Jr. had been shot near his groin, possibly by a fellow officer, and collapsed in a pool of blood at the corner of Dexter Avenue and Laurel Street. One civilian witness, who asked not to be identified, said there were police officers positioned behind Donohue and they appeared to be firing in his direction. The Middlesex district attorney’s office is investigating.

Two Harvard University officers, Ryan Stanton and Michael Rea, used tourniquets to stanch the flow of blood from Donohue’s wound. Others officers also assisted.

“So they immediately go to the aid of that officer — the first officer who was there and a number of other officers,’’ said Watertown Police Chief Edward Deveau. “This guy is bleeding out.”

Mullen said he heard desperate voices screaming in the night: “We’re losing him. We’re losing him. Get an ambulance here. Now.”

A dazed Jeffrey Ryan stumbled out on his porch on Dexter Avenue to witness the frantic efforts to save Donohue, 10 feet away in his driveway.

“I ask what I could do to help,” he recalled. “They said ‘Get towels, for tourniquets.’ My wife brought out a handful of towels. They did a great job saving his life.”

In Watertown, all firefighters are trained as EMTs. They are instructed to never enter an unsecured location: If you’re hurt, you’re useless.

“We would never drive into a shooting scene,’’ Watertown Fire Chief Mario A. Orangio would later say. “That’s a Bruce Willis movie scene.’’

But as their rig rolled down Laurel Street, Caruso and Menton tore up that book. At the scene, Caruso went to the rear of the ambulance to retrieve the stretcher, but Donohue had already been carried from the driveway and into the back for treatment. The stretcher never left the truck.

“We need to get him out of here!’’ police officers, shrouded in post-midnight darkness, shouted. “He’s bleeding bad! We need to go!’’

Donohue was bleeding profusely. He had a three-quarter inch bullet wound at the top of his right thigh. He had no pulse. His eyes were open. His color was gray.

“He was deceased,’’ is how Donahue looked to Caruso.

Caruso ripped Donohue’s blood-soaked pants apart, desperate to find the source of the bleeding. He grabbed two multi-trauma dressings, big gauze pads, and pushed them into Donohue’s wound.

Menton provided breathing for the breathless patient, using a “BVM,’’ a bag valve mask, that sent puffs of air into Donohue’s lungs.

Alongside them, Trooper Dumont, who had jumped aboard, began performing chest compressions.

“We need a driver! We need a driver!” officers outside shouted. Moments later the ambulance lurched forward.

In the front of the cab was Tim Menton. The Watertown officer who moments before had been in the street shootout now sped to Mount Auburn Hospital, the closest emergency room available. Within minutes, the rescue truck arrived at Mount Auburn.

“If we didn’t have three people in the back of the truck, I don’t know how it would have worked,’’ Pat Menton said, “because we were each doing a vital thing to save his life. We had to go.’’

FRIDAY, 1 A.M.

The search for Dzhokhar

Dzhokhar Tsarnaev’s wild ride from the shootout scene was a short one.

He abandoned the bullet-riddled Mercedes-Benz SUV — whose hijacking would be his undoing — at the corner of Spruce and Lincoln streets. It was only about a half-mile away from where his brother lay.

Officers may have been delayed in their pursuit of Dzhokhar as they tended to Donohue, but they didn’t wait long.

Three cruisers sped after Dzhokhar, two local officers and a state trooper, Captain Dupuis of Watertown said.

“He had a little head start on us,’’ he said.

“We have all of Greater Boston coming and I have other officers who have come in and everybody else is pursuing it,’’ Watertown Chief Deveau said. “Everybody always asks: How did the guy get away? Well, he got away from my six or seven guys because they had another mission at that point. And he didn’t go very far.’’

For a while at least, it was far enough.

As Dzhokhar careened down hilly Spruce Street, Max Kerman was getting ready for bed. He heard the distant gunfire and stepped out on the second-floor porch to investigate — just as the SUV came into sight.

“I see this black SUV come flying up the hill, with its front end damaged, one headlight out and the windows on the passenger side blown out,” he said. “There’s a tight curve at the top of the hill, but he didn’t even slow down. I’m surprised he didn’t hit parked cars.”

Kerman, 25, estimated it was 45 seconds before an unmarked police cruiser with lights flashing came up the hill, then slowed down in front of his house. “So I’m screaming down at the cop, ‘Keep going! Keep going!’ pointing down the street,” Kerman said.

Within minutes, scores of police swarmed the neighborhood. They did a room-by-room search of Austin Lin’s house on Spruce Street before evacuating Lin and his grandmother to the police station. The residents of six other nearby houses were also hustled out of their homes in bathrobes and sweatpants—most of them clutching cellphones — for the night.

“They had SWAT teams, dogs, and the National Guard going through backyards and checking basements and garages,” said Mary Karaguesian, who watched from her home, also on Spruce Street. “But they didn’t find him.”

Kerman said police swept the backyard of his house four or five times.

“They were really thorough,” he said. And careful.

Indeed, when officers located the battered SUV, they approached it with caution. Could the suspect be hiding inside? Was it booby trapped?

“Eventually they determined there was nobody in the car,’’ Dupuis said. “Then they started looking through some back yards to see which way he may have gone. At that point, with all the bombs and weaponry that they had, they just pulled back and regrouped.’’

Chief Deveau said the fleeing Tsarnaev could hear the sirens and see the lights of his pursuers. Cruisers approaching down Spruce Street forced him to bail. And he said the suspect had an added advantage: He was able to exploit out-of-town officers’ unfamiliarity with Watertown.

“It wasn’t like everybody just watched him go down the street,’’ Deveau said. “But at the same time, one of the disadvantages that we have is the rest of the officers aren’t familiar [with the area]. We know Watertown. My guys know Watertown. One of my officers literally had to drive the ambulance to the hospital because nobody else knew how to get there.

“So when they’re yelling he’s on whatever street, 95 percent of the people that are here — if not more — have no idea what that street means.’’

Later, police would recover only a handgun and a BB gun from the shooting scene. But in the heat of the pursuit, officers assumed he was armed and dangerous and possibly wearing a suicide vest. “They were using extreme caution,’’ the police chief said, “because they didn’t want to see an officer blown up.’’

Deveau said canine search teams were called in. “We had a lot of dogs here,’’ he said. “We had Boston, State Police, I think some municipal dogs and I know the federal people had dogs here, too. So we tried to do the tracking. . . . . But you can imagine there’s so many people there, the scent or whatever was just trampled.’’

Officers found blood and urine at a nearby house, but still, the trail dried up.

Within a half hour of Dzhokhar’s escape, Boston Police Superintendent Evans was in Watertown. It had already been a long day for the 32-year veteran of the Boston Police Department, who earlier in the week had completed his 18th Boston Marathon, crossing Boylston Street’s yellow-and-blue finish line about an hour before the twin blasts.

“They said, ‘Billy, you’re going to handle the street,’ ” said Evans. “It was an ugly scene. Shell casings. Pressure casing bomb remnants.’’

In fact, Laurel Street became one of the most complex crime scenes in the history of the Massachusetts State Police, officials said. More than 250 shell casings and several IEDs — exploded and not — would be recovered throughout the neighborhood. Among the items recovered were the remains of a pressure cooker bomb that had detonated into the side of a parked car.

A command post was quickly established at the Arsenal Mall. It was the beginning of a day of urgent searches, heart-stopping false alarms, and — for a time — a dreadful sense that Tsarnaev had somehow eluded them.

Laurel Street had to be evacuated. MBTA buses were summoned. A stop sign riddled with bullet holes was removed from the corner of Laurel Street and Dexter Avenue.

Anyone up and about in the early morning hours was searched by police. And a more systemic search grid was laid out.

The area was divided into five sectors. Color-coded maps were eventually drawn up. Before that, they were guided instead by Google images to put the lines together and define the sectors.

Deveau said it is unclear how many homes were searched. Linskey, the Boston chief, said “hundreds’’ but could not be more specific.

“We were trying to do 20 blocks around [Tsarnaev’s] vehicle,’’ Linskey said.

Authorities were concerned that the younger alleged bomber had remote-controlled improvised explosive devices and that he would double back through a neighborhood back yard. They monitored anything that moved.

At 1:16 a.m., the police came across a man who seemed suspicious. They stripped his clothes off and briefly cuffed him before they learned he lived on the street, Linskey said.

But the more intense, detailed search would wait for dawn.

FRIDAY, 6 A.M.

Shelter in place

By now, Watertown resembled an armed camp. Armored trucks rumbled through its streets. Police by the busload and an array of tactical units assembled at the Arsenal Mall.

DARREN McCOLLESTER/GETTY IMAGES

The day began with one suspect dead, a region locked down, andamassive manhunt underway.

Governor Patrick was briefed hourly throughout the night. Subway service was interrupted. As he raced from his home in Milton to the command center in Watertown, the governor received reports that a taxi that had been in Watertown was suspected of carrying explosive devices. Stopped in the Fenway, it turned out to be a false alarm. There were other reports of a police pursuit at South Station and another at the federal courthouse on the waterfront.

“It was coming fast and furious,’’ Patrick said. “You’re making judgments. We made the decision to suspend service on the T and to extend to Boston the request to stay indoors.’’

It was an extraordinary order and within moments it ricocheted around the world: A major American city is now under lockdown. Wall-to-wall television coverage showed battalions of helmeted police officers in Watertown and the nearly empty streets of what otherwise would have been the bustling streets of Boston.

Alben, the State Police colonel, promised Patrick that the tactical units would not stop until the younger of the alleged bombers was in custody.

“We are going to start going house to house on every single street,’’ Alben recalled. “This is something that is easier said than done. We’re going to knock on every person’s door and hope to have a conversation and ask to go into the house.’’

More dogs were brought in. Watertown issued reverse 911 alerts to residents about what was happening outside their windows. People began tweeting about what was going on in their backyards.

At dawn, five tactical teams were at work, each with 16 to 33 members.

The tactical units and other officers knocked on hundreds of doors to ask if anything seemed amiss and searched room-by-room if requested. They checked yards, sheds and other structures, including Sarina Tcherepnin’s barn next to her rambling yellow house on Franklin Street.

She told the officers around 9 a.m. that the doors are usually unlocked. It looked like a good hiding place. In fact, it was around the corner from where Tsarnaev was eventually found.

The officers searched it, but Tcherepnin said she and her husband had to suggest that they also examine the cellar of the barn.

“They were great,’’ she said. “But they didn’t look in the basement of the barn, which would seem an obvious place.’’

Across the street at Robert Vercollone’s green Victorian house, which the software consultant was remodeling, a State Police tactical officer aimed a gun at his pick-up truck loaded with yard trash. On the other side of the house, an officer pointed a flashlight at the latticework underneath his porch, which had a gaping hole because of ongoing plumbing repairs.

Aram Boghosian for The Boston Globe

Troopers stopped a vehicle at Carver Road and Belmont Street in Watertown on a tense Friday, as tensions ran high.

“It’s the perfect size for somebody to crawl through,” said Vercollone. “But he didn’t poke around any further.”

Residents universally praised the officers for being polite and helping to maintain a quiet calm over a panicked neighborhood.

People offered water and oranges to members of tactical teams. Though locked indoors, residents tried to carry on as normally as possible, planning a bar mitzvah, taking a nap, keeping little kids who were on school vacation away from televisions with scary images of the manhunt.

Throughout the day, there were multiple false alarms, at times interrupting search work as tactical units were called in to clear the area.

“A woman sends a text saying she was being held against her will by a man,’’ Superintendent Evans said. “It turns out she had some psychological issues. So we were running around all day long for suspicious people — residents hearing footsteps upstairs that weren’t supposed to be there.’’

Other dead-end tips were chased: Someone was running into a home on Oak Street; a 65-year-old man had a suspicious device on Arsenal Street; a man speaking Russian near reporters crossed a secure line.

“At least a dozen [times] just inside the perimeter, and then at the same time in the command post, we’re hearing different stuff that didn’t turn out to be accurate but you have to run it down,’’ said Deveau.

As evening approached, the Watertown chief worried.

“I’m not sure he’s gone . . . Did we have another carjacking that we didn’t get reports of?” Deveau said. “My other concern was that it was going to be dark in an hour and a half or two and it was going to give him a chance to move again if he was still here.”

By 5 p.m., with Dzhokhar still at large, Evans and his troops were feeling the weight of their work. They were living off bottled water and granola bars and — occasionally — searching for the nearest bathroom

“One poor old lady, I said, ‘Can I use your bathroom?’ She said, ‘It’s cluttered.’

“My mood was, we hadn’t finished our job,’’ Evans said. “Some of my officers were calling for release. I said, ‘Let’s hang in there. Let’s hang in there.’ ”

Evans was aware of the image of such a large police presence — by one estimate more than 1,000 officers — on the streets of an American city.

“We wanted to make sure people weren’t intimidated by us,’’ said Evans, saying they tried to put people at ease. “We were very careful about not upsetting the public. I felt awful when I looked at some people we asked to leave their house. . . . They were intimidated. We had weaponry, armored cars. We must have been very intimidating.’’

And then, not far away, a group of public officials walked toward a cluster of reporters.

FRIDAY, 6 P.M.

Capture

There was a tight-lipped blend of determination and disappointment as Governor Patrick stepped up to a bouquet of microphones as evening took hold. The Commonwealth’s capital had been essentially locked down. Streets were empty. Trains and subways cars stood still. The Red Sox and Bruins had called their games off. The Big Apple Circus was in town, but there were no clowns or elephants or trapeze acrobatics.

Lucy Lovrien

SWAT teams prepared to raid the Franklin Street yard where the suspect was believed to be hiding in a boat.

But the dragnet had come up empty.

And now the governor was lifting the so-called “shelter in place’’ order. Remain vigilant, he advised. But life’s routine, even with nerves rubbed raw, had to resume.

It was hardly a cavalier decision. Earlier in the day, he had spoken with Obama, who had already pledged the full support of the US government.

Patrick recalled the conversation: “He said, ‘Have you thought about the shelter in place request?’ And I said yes. He said, ‘There’s a point at which it’s no good anymore.’ That’s a paraphrase. ‘There’s a point at which we should consider whether it should continue.’ I said, ‘I get that.’ ’’

Authorities appeared before a battery of cameras and microphones in Watertown to announce the decision to rescind the order, telling residents they should continue living their lives — though with extra vigilance.

Blocks away, homeowner David Henneberry on Franklin Street took advantage of the relaxed civil alert to take in some fresh air in his driveway, where his shrink-wrapped boat, the Slip Away II, still held the promise of a summertime idyll.

Each year Henneberry delights a half dozen or so children from the Perkins School for the Blind who clamber aboard his vessel for a leisurely voyage up and down the Charles River, said Vercollone, the Franklin Street neighbor whose girlfriend works at the school.

But now something was amiss. The straps weren’t quite right. The pads seemed somehow askew. And the boat owner was meticulous

Henneberry, a former telephone company technician, climbed a ladder and peeked inside.

There was blood. A lot of blood. And on the other side of the boat’s engine box there was a body.

Within moments of Henneberry’s 911 call there, SWAT team members walked up neighbor Dumitru Ciuc’s driveway. There was a loud knock on the door and an officer’s urgent order: Get out now.

“We’re getting a report from Watertown of 67 Franklin Street,’’ an officer’s voice crackled over a police scanner. “They have a boat with blood on it and they believe someone’s on the boat. . . . I’ve got the owner of the house here. He says that there’s a body in the boat.’’

Seventeen hours after the bullets-and-bombs shootout on Laurel Street, less than a mile away, another Watertown neighborhood was under intense siege. After a massive search, the suspect would be found less than a quarter mile from where he abandoned the black SUV.

Lucas Jackson/Reuters

The cover of the boat, the Slip Away II, was torn away in an attempt to extract Dzhokhar Tsarnaev.

A command post was established. A battalion of heavily armed and Kevlar-vested officers moved in. A State Police helicopter, a Eurocopter TwinStar piloted by Trooper Mark Spencer, was sent aloft equipped with a thermal imaging camera.

Patrick, who was headed home with take-out Thai food he had picked up in Quincy, quickly called his wife with a change of plans. “Meet me at Exit 10 and I will give you the food,’’ he said. He changed course and headed for Watertown.

What happened next is captured on police radio frequencies and State Police video of infrared images of the heat signatures coming from the man beneath the tarp.

After police scanners chirped with news that the person in the boat was moving, an officer announced that the vessel was surrounded on all sides and the person on it was “trying to poke a hole’’ through the tarp. Presently, there was a fusillade of gunfire.

Superintendent Evans instantly barked: “Hold your fire!’’

“What happens in those situations [is], once one fires, others fire,’’ Evans later explained. “We’ve got the guy. I didn’t want something to happen to our guys and didn’t want him to be killed. I knew we had it under control.’’

Ambulances were summoned, as was a tactical response team that had already spent its day performing house-to-house searches along Dexter Avenue.

At 6:30 p.m. the team had gone off duty and was back at MBTA Police headquarters on Southampton Street in Boston when units began converging at Henneberry’s house.

Within minutes, they returned to Franklin Street, where Officer Jeff Campbell, 45, approached the federal tactical officer in charge. He was hoping for a quick briefing — and got something more.

“Can you gather a team and be ready in 5 minutes?” the federal commander asked.

“Roger that,” Campbell replied.

Meantime, other officers with rifles took up sniper positions around and above the house. Alerts from the air confirmed continued movement in the boat.

“We’re putting some dummy rounds into the boat. So nobody should be firing, OK,’’ one commander shouted. “These are just dummy rounds being fired into the boat so everybody hold fire, all right?’’

There were warnings about the dangers of booby traps, about 20 gallons on gasoline aboard the boat, and the perils of crossfire in close quarters. Scene commanders did their best to shoo away the knots of eager officers who had clogged the area. Tactical is taking over, they broadcast. Everyone else, leave the scene.

A heavily armored vehicle approached the boat.

Bureau of Alcohol, Tobacco, Firearms and Explosives

A wounded Tsarnaev was handcuffed, bringing the nightmare of a daylong manhunt to a dramatic end.

“State Police are putting their extended ram on the front of their armor,’’ officers were told. “They’re going to try to make their way into the back yard. They’re going to try to rip the tarp off the boat. . . . If they’re able to successfully rip that off, he’ll be fully exposed.’’

Dzhokhar was spotted on his back. “He picked his arm up. His arm is covered in blood,’’ an officer announced. “He put it in the air. He brought it back down to his chest. He’s on the left side of the boat when you’re looking at it from the driveway.’’

Officers were ordered to keep off their radios.

“We know you’re in there,’’ police yelled. “Come out on your own terms. Come out with your hands up.’’

Dzhokhar’s capture was orchestrated by the state police SWAT and FBI’s hostage rescue teams under the direction of a blue jean-wearing FBI agent sent from headquarters in Virginia. His name was never released to the public, but he remained calm in the command trailer, spitting tobacco as he oversaw the high stakes negotiations.

“For what [the suspect] put us through over last four days — devices, handguns — we had to assume he possibly had bombs on there [the boat],’’ said Evans. “He might have had a suicide vest. He might have had handguns. The poking at the tarp indicated to me he was either trying to break the tarp or poke a firearm through there.

“We didn’t know what he had, but given what he did at the scene of the Marathon, given what he did during the shoot-out, and given what he did to the MIT officer, we knew we were dealing with a serious terrorist here who had weapons to the max. We didn’t know what we were going to walk into.”

In short order, the tactical team began its advance on the boat.

A Malden Police SWAT officer led the way, holding a black Kevlar shield. Behind him, in single file, was another Malden Police SWAT officer, carrying an M4 carbine. They were followed by Transit SWAT officers Campbell, Saro Thompson, 33, Kenny Tran, 35, and Brian Harer, 44. All wore Kevlar helmets and carried submachine guns.

The team headed down the driveway toward the boat at a normal walking pace. An earpiece tethered them to a command post.

“Watch his hands,” they reminded each other.

As daylight slowly faded, Harer shouted: “Show us your hands and get down off the boat.”

Dzhokhar was ordered to lift his shirt, but did not respond.

“To me he was refusing our command,” Campbell would later recall. “That meant we had to go get him.”

Dzhokhar was lying on his stomach, straddling the side of the boat, wearing jeans and a hoodie. His left arm and left leg hung over the boat’s side. He appeared to struggle for consciousness.

It was unclear whether he had firearms or explosive devices or had rigged the boat to explode. His face was bloody. He appeared exhausted, in need of urgent medical care.

As team members studied his movements, Dzhokhar’s hands shot up in a surrender gesture. Still, he rocked left and right, his right hand intermittently dipping out of view.

Is he reaching for a weapon? Campbell wondered.

The team was now within eight feet. And when both hands appeared again, the team struck.

“We broke from behind the shield and went after him,” Campbell recalled. “It took only a second or two to get him off the boat.”

Campbell reached up over his head — at a height of about 7 feet — and using both hands he grabbed Dzhokhar’s left arm and left leg and in one motion hauled him down to the grassy ground.

Dzhokhar, on his stomach, was frisked and then flipped over.

Aram Boghosian for The Boston Globe

After the arrest, police officers were cheered as heroes. One of them posed with admirers on Boston Common.

Blood covered the left side of his face. His left ear was severely wounded and bleeding. There was a bleeding cut — a 2-inch slice as if hit by shrapnel — on the front of his neck, just below his chin. Blood flowed from a substantial wound in his thigh.

He appeared unarmed.

Dzhokhar’s body was flipped again, back on his stomach. Thompson grabbed hold of his left arm, Tran his right.

Thompson snapped handcuffs on his wrists.

Click.

At about 8:45 p.m., the radio crackled again.

“He’s in custody! He’s in custody!’’

The SWAT team called for a medic and an ambulance. Then, amidst the police radio traffic, the Boston mayor’s voice: “People of Boston are proud of you,” Menino said.

An ambulance bearing the wounded Tsarnaev soon pulled into the street, flanked on all sides by police cars. Along their route, residents came out to line the streets, clapping and cheering the departing officers. For them, it was time to finally exhale.

For others, the road ahead remained long and uncertain. From the hospital where their daughter still lay wounded, the family of Martin Richard sent a statement to the world, expressing gratitude in the midst of endless grief.

“None of this will bring our beloved Martin back, or reverse the injuries these men inflicted,” it said. “We continue to pray for healing and for comfort on the long road that lies ahead.”

This story was written by Jenna Russell and Thomas Farragher. It was based on more than 100 interviews with police, government officials, and witnesses of the week’s events. The reporting was conducted by Globe staff writers Andrea Estes, Sean P. Murphy, Shelley Murphy, Matt Carroll, Jonathan Saltzman, Michael Rezendes, David Abel, Jenn Abelson, Brian Ballou, Todd Wallack, Maria Cramer, Maria Sacchetti, Andrew Caffrey, Bill Greene, Michael Levenson, Stephanie Ebbert, Akilah Johnson, Kevin Cullen, Eric Moskowitz, Jenifer B. McKim, Wes Lowery, and Milton Valencia. Globe correspondents Evan Allen, Derek J. Anderson, and Alli Knothe also contributed.
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