Investigators began piecing together the story behind a mass shooting on Monday at the US Navy Yard in Washington that left 13 people dead, including the gunman, and several wounded.

The FBI was working to “piece together the movements and activities of Aaron Alexis,” 34, identified as the assailant, said Valerie Parlave, assistant director in-charge of the Washington FBI field office.

Alexis appeared to be a troubled military veteran involved in previous shootings, although never formally charged, according to the picture emerging from news reports.

Alexis was “the single and sole person responsible for the loss of life,” Washington Police Chief Cathy L Lanier said late on Monday, after earlier reports described two other possible shooters.

The victims ranged in age from 46 to 73, Washington Mayor Vincent Gray told reporters.

Eight people were injured, of whom three had been shot, he said, correcting earlier reports that up to 12 were wounded by gunfire.

The sprawling naval base on the Anacostia River is about 2 kilometres south of the US Capitol.

Gunman had valid pass

The FBI’s Parlave said Alexis had “legitimate access” to the Navy Yard and used a valid pass acquired as a civilian contractor.

He was armed with an assault rifle and a handgun, news reports quoted law enforcement officials as saying.

Military records show Alexis was an active Navy Reserve member from 2007-11 and was discharged after reaching the rank of aviation electrician’s mate.

He was suspected of shooting out the tyres of a construction truck next to his home in Seattle in 2004. According to a report issued by the Seattle Police Department, Alexis told police that it was an anger-fuelled “blackout.” He allegedly had been involved in the events of September 11, 2001 which had “disturbed him,” his father said.

Alexis was a practicing Buddhist and spoke Thai fluently, according to news reports.

Michael Riotrovato, an acquaintance in Fort Worth, told CNN he had worked in Tokyo on an assignment for his civilian contractor employer, but complained that he hadn’t been paid.

The shooting began around 8:20 am (1220 GMT) inside the Naval Sea Systems Command headquarters, where about 3,000 people work.

Alexis was shot dead after a running firefight through the building, Ms. Lanier said, adding that authorities did not believe Alexis took his own life.

Obama decries shooting

US President Barack Obama decried “yet another mass shooting.” “These are men and women who were going to work, doing their job protecting all of us,” he said. “They’re patriots, and they know the dangers of serving abroad, but today they faced the unimaginable violence that they wouldn’t have expected here at home.”

Mr. Obama called for a “seamless” probe of the incident by local and federal authorities: “As this investigation moves forward, we will do everything in our power to make sure whoever carried out this cowardly act is held responsible.”

The President also ordered flags lowered to half mast until Friday and cancelled a concert on Monday evening at the White House.

A police officer who suffered severe leg injuries, a woman who was wounded in the shoulder, and another with injuries to the scalp and hand, were being treated at Washington Hospital Centre, chief medical officer Janis Orlowski said.

“Their chances for survival are very good,” Orlowski said.

The Navy issued an “order to account” for all facilities in the capital area, meaning that all uniformed personnel, their families and civilian workers are counted, using an online mustering website.

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