(NECN: Justin Michaels) - Thirteen people are dead, including the gunman and eight are wounded following a mass shooting at the Washington Navy Yard.
The gunman has been identified as 34-year-old Aaron Alexis, a civilian contractor and former Navy reservist.
The FBI says Alexis had legitimate access to the Navy Yard, and used a valid pass to get in.
Authorities spent much of Monday searching for a second suspect, but, late Monday night, they said they were convinced the attack was the work of a lone gunman.
Mayor Vincent Gray said, “We don't have any evidence, any indication at this stage that there was yet another shooter, even though we haven't completely ruled that out."
None of the victims were active duty military personnel.
Some were civilian employees and contractors, ranging in age from 46 to 73.
Gray said, "We don't know what the motive is at this stage, we were asked earlier if there was likely terrorism involved. We have no indication of that, but, we haven't ruled it out."
Alexis had shown flashes of anger before.
He was arrested in 2004 in Seattle for shooting out the tires of another man's car, in what he told police was an anger-fueled blackout.
His father told police his son had post-traumatic stress, brought on by the 9/11 terror attacks.
Witnesses say Alexis stood on a fourth floor balcony firing down at workers in a food court.
Sean Carroll said, "I was running and then I heard gunshots and then holy cow, that's, I don’t know it could have been way above me, but I wasn't stopping to think you know, what was going on."
Police say Alexis brought a shotgun and then used an assault rifle he got from a guard he shot at the base.
Alexis was treated multiple times for psychological issues including sleep deprivation, anger and paranoia.
Most recently his treatment was performed at an unspecified veteran’s affairs hospital here in New England.