The attack at a single building at the highly secured Navy Yard unfolded about 8:20 a.m. in the heart of the U.S. capital, less than four miles (6.4 kilometers) from the White House and two miles (3.2 kilometers) from the Capitol.
It put all of Washington on edge and raised the specter of another well-coordinated terrorist strike or another attack from within, like Fort Hood.
The gunman, Aaron Alexis, a 34-year-old information technology employee and former Navy reservist whose last known address was in Fort Worth, Texas, died after a running gunbattle inside the building with police, investigators said.
He carried three weapons: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene, according to two federal law enforcement officials who spoke on condition of anonymity because they were not authorized to discuss the investigation.
For much of the day, authorities said they were looking for a possible second attacker who may have been disguised in an olive-drab military-style uniform.
But by late Monday night, they said they were convinced the attack was the work of a lone gunman, and the security lockdown around the area was eased.
"We do now feel comfortable that we have the single and sole person responsible for the loss of life inside the base today," Washington Police Chief Cathy Lanier said.
The FBI took charge of the investigation.
In addition to those killed, eight people were hurt three of them shot and wounded, according to the mayor. Those three were a police officer and two female civilians, authorities said. They were all expected to survive.
The dead ranged in age from 46 to 73, according to the mayor. A number of the victims were civilian employees and contractors, rather than active-duty military personnel, the police chief said.
At the time of the rampage, Alexis was an information technology employee with The Experts, a company that was a Defense Department subcontractor on a Navy-Marine Corps computer project, authorities said.
Valerie Parlave, head of the FBI's field office in Washington, said Alexis had legitimate access to the base as a defense contractor and used a valid pass.
Alexis had been a full-time Navy reservist from 2007 to early 2011, leaving as a petty officer third class, the Navy said. It did not say why he left. He had been an aviation electrician's mate with a unit in Fort Worth.
A convert to Buddhism who grew up in New York City, Alexis had had run-ins with the law over shooting incidents in 2004 and 2010 in Fort Worth and Seattle and was portrayed in police reports as seething with anger.
Witnesses on Monday described a gunman opening fire from a fourth-floor overlook, aiming down on people on the main floor, which includes a glass-walled cafeteria. Others said a gunman fired at them in a third-floor hallway.
Patricia Ward, a logistics-management specialist, said she was in the cafeteria getting breakfast.
"It was three gunshots straight in a row pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop, so it was like about a total of seven gunshots, and we just started running," Ward said.
The FBI would not give any details on the gunman's weaponry, but witnesses said the man they saw had a long gun which can mean a rifle or a shotgun.
The shooting quickly reignited the debate over gun control in the United States, but it was far from certain what the impact would be.
The politics of gun control have only gotten tougher since December's shooting at Connecticut's Sandy Hook Elementary School. That shooting, which killed 20 first-graders and six staffers, spurred Obama to propose stricter firearms laws.
White House spokesman Jay Carney on Monday reiterated the Obama administration's commitment to strengthening gun laws, including expanding background checks to sales online and at gun shows.
Gun owners, aided by their advocates at the National Rifle Association, the country's largest gun rights lobby, have successfully fought Obama's legislation, even though polls show broad support for tougher gun laws.
The Washington Navy Yard is a sprawling, 41-acre (16.6-hectare) labyrinth of buildings and streets protected by armed guards and metal detectors, and employees have to show their IDs at doors and gates. More than 18,000 people work there.
The rampage took place at Building 197, the headquarters for Naval Sea Systems Command, which buys, builds and maintains ships and submarines. About 3,000 people work at headquarters, many of them civilians.
Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and co-workers encountered a gunman in a long hallway on the third floor. The gunman was wearing all blue, he said.
"He just turned and started firing," Brundidge said.
Terrie Durham, an executive assistant with the same agency, said the gunman fired toward her and Brundidge.
"He aimed high and missed," she said. "He said nothing. As soon as I realized he was shooting, we just said, 'Get out of the building.'"
As emergency vehicles and law enforcement officers flooded streets around the complex, a helicopter hovered, nearby schools were locked down and airplanes at nearby Reagan National Airport were grounded so they would not interfere with law-enforcement choppers.
Security was tightened at other federal buildings. Senate officials shut down their side of the Capitol while authorities searched for the potential second attacker. The House of Representatives remained open.
After authorities determined late Monday that the available evidence indicated the deceased shooter acted alone, Sergeant at Arms Terrance Gainer said all operations at the Senate would return to normal Tuesday.
In the confusion, police said around midday that they were searching for two men who may have taken part in the attack one carrying a handgun and wearing a tan Navy-style uniform and a beret, the other armed with a long gun and wearing an olive-green uniform. Police said it was unclear if the men were members of the military.
But as the day wore, police dropped one person and then the other as suspects.
As tensions eased, Navy Yard employees were gradually released from the complex, and children were let out of their locked-down schools.
Adm. Jonathan Greenert, chief of naval operations, was at the base at the time the shooting began but was moved unharmed to a nearby military installation.
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Associated Press writers Jesse Holland, Stacy A. Anderson, Brian Witte and Ben Nuckols in Washington contributed to this report.
