Washington. A gunman who died after killing 12 people at a naval installation in Washington DC has been identified as a former US Navy serviceman.Aaron Alexis, 34, of Fort Worth, Texas, died in a gun battle with police during the rampage at Washington Navy Yard.Eight others were injured in the attack, which began at about 8:20 local time (12:20 GMT) on Monday.US President Barack Obama ordered flags be lowered to half mast at the White House and on Capitol Hill.He said he mourned "yet another mass shooting” and condemned the "cowardly act”. No motive is known for the attack, whose dozen victims ranged in age from 46 to 73, said the authorities.Police initially responded to reports of two other armed suspects, but after exhaustive searches Washington Police Chief Cathy Lanier said on Monday night they were all but certain that Alexis was the only gunman.Witnesses said the suspect sprayed bullets in a hallway and fired from a balcony down on to workers in an atrium at the heavily secured installation in the US capital.He was reportedly armed with an AR-15 semi-automatic rifle, a shotgun and a handgun that he took from a police officer at the scene.Chief Lanier told reporters that the attacker "was engaged in shooting with police officers” when he died. "There is no question he would have kept shooting,” she added.Valerie Parlave of the FBI said that the suspect - who was employed by The Experts, a Hewlett-Packard subcontractor that was working on a US Navy intranet network - had a valid pass to enter Washington Navy Yard.Alexis was formerly a petty officer 3rd class who served full-time in the naval reserve from 2007-11, said the US Navy.Agencies.