Authorities said they were looking for a possible second attacker who may have been disguised in an olive-drab military-style uniform.
But as the day wore on and night fell, the rampage increasingly appeared to be the work of a lone gunman, and Navy Yard employees were gradually being released from the complex and children were let out of their locked-down schools.
Investigators said they had not established a motive for the attack, which unfolded at about 8:20 a.m. in the heart of the nation’s capital, less than four miles from the White House and two miles from the Capitol.
As for whether it may have been a terrorist attack, Mayor Vincent Gray said: “We don’t have any reason to think that at this stage.” But he said the possibility had not been ruled out.
It was the deadliest shooting rampage at a U.S.-based military installation since Maj. Nidal Hasan killed 13 people and wounded more than 30 others in 2009 at Fort Hood in Texas. He was convicted last month and sentenced to death.
President Barack Obama lamented yet another mass shooting in the U.S. that he said took the lives of American patriots. He promised to make sure “whoever carried out this cowardly act is held responsible.”
The FBI took charge of the investigation and identified the gunman killed in the attack as 34-year-old Aaron Alexis of Texas. He died after a running gunbattle with police, investigators said.
Authorities were investigating how he got onto the base. Officials said he may have had a badge that allowed access.
At the time of the rampage, he was working in information technology with a company that was a Defense Department subcontractor.
Alexis was 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, Texas.
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.
In addition to those killed, more than a dozen people were hurt, including a police officer and two female civilians who were shot and wounded. They were all expected to survive.
The Washington Navy Yard is a sprawling labyrinth of buildings and streets protected by armed guards and metal detectors, and employees have to show their IDs at doors and gates to come and go. About 20,000 people work there.
The rampage took place at Building 197, the headquarters for Naval Sea Systems Command, which buys, builds and maintains ships, submarines and combat systems. About 3,000 people work at headquarters, many of them civilians.
Witnesses described a gunman opening fire from a fourth-floor overlook, aiming down on people in the cafeteria on the main floor. Others said a gunman fired at them in a third-floor hallway.
Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other 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.’”