They make their way onto Boylston from Gloucester Street, turning left and into the crowds, to the spots where they are to drop their backpacks containing homemade bombs. Tamerlan Tsarnaev leads the way, heading toward the finish line, leaving his younger brother outside the Forum.
There, Dzhokhar Tsarnaev leans into a crowd, his backpack no longer slung over his shoulder. It is at his feet, an FBI agent testified, right behind several children.
Moments later, there are flashes and explosions, and Tamerlan Tsarnaev calmly walks back down Boylston, and north on Exeter. His younger brother runs from the scene, turning north on Fairfield.
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The movements of the Tsarnaev brothers in the minutes just before and after the deadly April 15, 2013, explosions were traced in detail for a federal jury Monday, pieced together from surveillance videos and photos, patched into a grim video timeline.
Jurors watched the presentation after earlier hearing from Jessica Kensky, who described how one of the bombs threw her into the air, causing injuries that would ultimately lead to the amputation of both legs.
“These are war wounds,” she told jurors from a wheelchair on the witness stand. She was accompanied by her service dog, Rescue.
And Danling Zhou described to jurors her final moments with Lingzi Lu, a graduate student from Boston University and one of the three people killed in the explosions.
“I tried to calm her down,” a tearful Zhou, who is also from China, told jurors. “I can [see] her injury. I can’t hear, so I can only [see], she’s yelling.”
Zhou, a fellow BU grad student at the time, was rushed to a hospital, where she remained for nearly a month. Her friends and caregivers waited for days before telling her that Lu died in the explosions.
“Everybody knew that earlier, but they didn’t tell me, because they were trying to protect me,” she said, wiping tears from her eyes.
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Monday was the third day of testimony in the Boston Marathon bombing trial, and jurors have heard from 27 witnesses who described injuries they suffered in the explosions, or their efforts to help others who were hurt. Law enforcement officials have testified about their response to the bombing, and the investigation into the explosions.
Also on Monday, an FBI agent testified that Tsarnaev managed a secret Twitter account under the profile name Ghuraba, or stranger, in which he posted extremist views supporting Al Qaeda.
Listen to Anwar al Awlaki's (a shaheed iA) the here after series, you will gain an unbelievable amount of knowledge #islam #muslim
— Ghuraba (@Al_firdausiA) March 11, 2013
Prosecutors say the 21-year-old and his older brother were inspired by Al Qaeda to carry out the bombing at the Marathon as retribution for US military involvement in Muslim lands.
Tamerlan Tsarnaev was killed four days after the bombings in a confrontation with police in Watertown. The brothers also allegedly shot and killed a Massachusetts Institute of Technology police officer earlier that night.
Dzhokhar Tsarnaev faces the possibility of the death penalty. Though his lawyers have admitted that he took part in the bombings, he has pleaded not guilty and his lawyers have sought to use the trial to convince jurors that he is less culpable — that his older brother was the mastermind — and their client is less deserving of death.
In making their case, prosecutors so far have called witnesses to detail their recollection of the bombing and detail their injuries; and law enforcement witnesses to testify about the response and investigation.
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James Hooley, head of Boston Emergency Medical Services, testified Monday that 118 people were rushed by ambulance to local hospitals in the minutes after the bombings. Officials set up a triage area where they classified which victims were most in need of care.
Boston police Superintendent-in-Chief William Gross testified that police commanders dispatched all available resources to the bombing scene, and the department at one point used police wagons to transport victims.
One commander had shouted into the police radio, “Stop the race; give me everything you have to Boylston Street,” Gross recalled.
Train service to the area was stopped, and police swept the area in search of other explosive devices. The scene was cleared, with all victims removed, within 22 minutes, Gross testified.
“We had no idea who planted the bombs, who detonated them, whether it was one person or two people, or an entire cell,” Gross said. “Everything was at a standstill until we could figure out what happened. Again, a standstill in Boston.”
Bystanders also tended to victims. Dr. James Bath, a general practitioner from Charlestown, was on nearby Newbury Street when he heard the explosions.
As he approached Boylston Street, he testified that he saw people fleeing the area, including one man in tattered clothes who shouted, “All my friends are dead; I need to get out of here.”
He sought to help some of the victims, including Lu, but found her in agonal breathing, what he called “the body’s last gasp, essentially, to take in air.”
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Matthew Patterson, a Lynn firefighter who had been at a private party at Abe and Louie’s on Boylston Street, testified that the “energy release” told him that the explosions he had heard came from bombs, and he rushed toward the scene. One of the first people he helped was Jane Richard, the young girl whose 8-year-old brother Martin Richard was killed in the explosions. Photos of Patterson carrying Jane Richard from the scene were shown to the jury.
“The scene was in chaos,” he testified.
In the surveillance footage of the Tsarnaev brothers’ movements, Dzhokhar Tsarnaev could be seen in the Marathon crowd, rushing away from the explosion he set off.
The jury was also shown a video of Tsarnaev, 23 minutes after the explosions, at a Whole Foods in Cambridge, taking a moment to decide which milk to buy. He settled on a half-gallon that he paid for with a $20 bill.
The next night, Tsarnaev was at the gym at the University of Massachusetts at Dartmouth with a friend, which was also captured on video.
Milton J. Valencia can be reached at mvalencia@
globe.com. Follow him on Twitter @miltonvalencia.