WASHINGTON — A worker for a defense contractor shot to death at least a dozen people as the workday began at the Washington Navy Yard on Monday, creating an improbable morning of horror at a military facility with armed guards at every gate and leaving investigators seeking clues about what spurred the attack.
The suspected gunman, identified by the FBI as Aaron Alexis, 34, of Fort Worth, was a former Navy reservist who reportedly had previous run-ins with police over firearms. Police said Alexis, who used a valid badge to enter the complex, was killed in a fierce gunbattle with officers.
‘‘It was three gunshots straight in a row — pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop,’’ Patricia Ward, a logistics-management specialist who was in the cafeteria getting breakfast, told the Associated Press. “We just started running.’’
Police Chief Cathy L. Lanier had said early in the day that investigators were looking for a possible second attacker who may have been disguised in a military-style uniform, but by nightfall the attack increasingly appeared to be the work of a single gunman.
Officials said that in addition to those killed, about eight people were hurt, several critically. At least two police officers were among those wounded. Authorities late Monday began releasing the names of the dead and wounded, but some family members were still awaiting word. The dead ranged in age from 46 to 73 years old.
Mayor Vincent C. Gray said no motive has been determined. He said there is no reason to believe it was an act of terrorism, though he added that he could not rule it out.
The attack was the deadliest mass shooting on a military installation in the United States since the tragedy at Fort Hood, Texas, in 2009.
Alexis was a full-time reservist from 2007 to early 2011, leaving as a petty officer third class on a general discharge, a designation that usually signals a problem in his record.
At the time of the rampage, Alexis was working as a Defense Department contractor, but it was not clear if the information technology worker was assigned at the Naval Yard, according to two defense officials who spoke on condition of anonymity because they were not authorized to discuss the matter publicly.
He was also pursuing a bachelor’s degree in aeronautics online with Embry-Riddle Aeronautical University, the school said. He started classes in July 2012.
The carnage began about 8:15 a.m. Investigators said Alexis shot a security guard, most likely with a shotgun he bought in Lorton, Va., outside Building 197 at the Navy Yard, officially known as Naval Sea Systems Command. About 3,000 people work in the building.
The suspect took the guard’s handgun before moving methodically through the interior, they said.
As people scattered for cover, they turned to text messages and office televisions in an effort to determine what was going on.
‘‘We were sort of in the dark,’’ said John Norquist, 52, a Fairfax, Va., lawyer who served as a civilian adviser in Afghanistan last year. ‘‘We were trained in active shooter scenarios.’’
Rick Mason, a program management analyst who is a civilian with the Navy, told the AP that a gunman was shooting from a fourth-floor overlook in the hallway outside his office. He said the gunman was aiming at people in the building’s first-floor cafeteria.
Terry Durham said that as she and co-workers were evacuating, she saw a man down the hall raise a rifle and fire toward them, hitting a wall.
“He didn’t say a word,” said her co-worker, Todd Brundage.
One man, who said he was at his desk on the second floor when the shooting began, recalled hearing a loud noise “like someone dropping an old metal desk.”
The man, who declined to give his name, said there was a pause, then several noises close together, and he realized the danger: “There’s a shooter in the building. I started walking toward the door and I heard people running down the hall.”
Employees described the chaos, as a fire alarm sounded and people shouted, “Where is he? Where is he?”
Gregory Dade, a Navy contractor, said he and a co-worker locked themselves in a second-floor office of Building 197 as soon as the shooting went on, in fits and stops. Dade, called it “terrifying.”
He heard a woman scream, glass crashing, and a series of gun shots. Then he heard shouting: “Get down! Get down! This is the police.”
Finally, about 11 a.m., he and a co-worker made a break for it. At an exit, they noticed a trail of blood running to the next building.
The full weight of Washington’s vast antiterrorism network converged on the area within minutes as local and federal law enforcement teamed to sweep the Navy Yard and the neighborhood along the Anacostia River.
The shootings threw the nation’s capital into turmoil, with police initially fearful two other gunmen might be on the loose.
People were warned to remain in their homes and those at offices on the naval base and in the surrounding neighborhood were told to stay put.
Flights were briefly halted at Reagan National Airport. Schools near the base were locked down. The Senate adjourned early, and people were not allowed to enter or leave much of the Capitol complex.
‘‘This has been a dark day,’’ said House Speaker John Boehner, Republican of Ohio.
The Washington Nationals, whose ballpark is close to the base, were told to stay away from the stadium during the search. The Red Cross set up an evacuation and meeting site at the ballpark’s parking lot for workers at the yard. The game against the Atlanta Braves was postponed. The official Major League Baseball description of the game was stark: ‘‘Postponed: Tragedy.’’
Among those injured was a D.C. police officer who was shot twice in the leg. He is expected to survive.
‘‘There’s no question [the gunman] would have kept shooting,’’ said Lanier, who declined to say how many shots were fired from start to finish. Police said they believe that Alexis also obtained an assault rifle once he was in the building, but it was unclear how.
Alexis grew up in Brooklyn with his mother, Sarah, and father, Anthony, according to his aunt Helen Weeks. “We haven’t seen him for years,” Weeks said of her nephew.
Alexis lived in Seattle in 2004 and 2005, according to public documents. In 2004, Seattle police said, Alexis was arrested and charged with shooting out the tires of another man’s vehicle in what he later described to detectives as an anger-fueled ‘‘blackout,’’ the Associated Press reported.
According to an account on the department’s website, two construction workers had parked their Honda Accord in the driveway of their worksite, next to a home where Alexis was staying. Police eventually arrested Alexis on a charge of malicious mischief.
When Alexis was still in the Reserve in September 2010, a neighbor in Fort Worth reported she had been nearly struck by a bullet shot from his downstairs apartment.
Fort Worth police questioned Alexis about the neighbor’s report; he admitted to firing his weapon but said he was cleaning his gun when it accidentally discharged. Alexis was arrested on suspicion of discharging a firearm within city limits, but the case was not pursued, the AP said.
President Obama expressed sympathy for the victims of the Navy Yard shooting and said justice must be sought. “I’ve made it clear to my team that we want the investigation to be seamless, so that local and federal authorities are working together,” he said.
The rampage was the single worst loss of life in the region since the 2001 attacks on the Pentagon killed almost 184 people.