Questions are being asked as to why the Washington massacre gunman Aaron Alexis, who had "serious mental health problems" and previous incidents with firearms, was able to hold official security clearance to work at the US Navy base complex where he killed 12 people.
Police said he had been hearing voices, and had contacted ex-servicemen's hospitals for psychological help.
Washington Correspondent Robert Moore reports on the seventh mass shooting of Barack Obama's presidency.
Police would not be drawn on any potential motive for the rampage, but said the Navy had not declared the defence contractor mentally unfit, which would have meant that his official security clearance Alexis held from his earlier time serving in the Navy Reserves.
Alexis used a valid pass to enter the complex. In the past he had complained about his time in the Navy, and told friends he had be the victim of racism and discrimination.
The FBI said he entered the complex with a shotgun bought lawfully in nearby Virginia, and obtained another handgun once inside. Police said they are convinced he acted alone.
Alexis worked with a company that was a Defence Department subcontractor on a Navy-Marine Corps computer project.
He had been a full-time Navy reservist from 2007 to 2011, and had a string of misconduct problems, but received an honourable discharge.
A recent convert to Buddhism who grew up in New York City, Alexis had had shooting incidents in 2004 and 2010 in Fort Worth and Seattle, police described him as "seething with anger."