story updated 9:15 p.m.
WASHINGTON (CBSDFW.COM/AP) — A former Navy reservist went on a shooting rampage Monday inside a building at the heavily secured Washington Navy Yard, firing from a balcony onto office workers in an atrium below, authorities and witnesses said. Thirteen people were killed, including the gunman.
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.
The attack unfolded 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.
Investigators said the motive was a mystery.
Mayor Vincent Gray said there was no indication it was a terrorist attack. But he said the possibility had not been ruled out.
It was the deadliest shooting at a military installation in the U.S. since Maj. Nidal Hasan killed 13 people and wounded more than 30 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 inside the building with police, investigators said.
At the time of the rampage, Alexis was working in information technology with a company that was a Defense Department subcontractor.
Authorities said he may have had a badge that enabled him to get onto the base, but they were still investigating.
Alexis was portrayed in police reports as seething with anger.
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.
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.
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.
Police 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.
CLICK BELOW FOR CONTINUING COVERAGE: