The Washington massacre gunman had serious mental problems and suffered fits of rage, but was still able to hold an official security clearance for the complex where he killed 12 people.
Aaron Alexis’s motive in the navy yard rampage remains a mystery but law enforcement officials said he had paranoia and a sleep disorder and was hearing voices in his head.
They also said there has been no connection to international or domestic terrorism, and investigators have found no manifesto or other writings suggesting a political or religious motivation.
The attack is unlikely to lead to tighter gun controls. Measures proposed during national outrage over the school shooting in Connecticut in December, when 20 children were killed, failed this year in Congress.
Family members said that Alexis, 34, who was shot dead by police, had been treated for his mental problems since August.
The navy had not declared its defence contract employee mentally unfit, which would have rescinded the security clearance Alexis had from his earlier time in the Navy Reserves.
He used a valid pass to get into the Washington Navy Yard. At the time of the shooting, Alexis was an employee with a company that was a Defence Department subcontractor on a Navy-Marine Corps computer project.
In the past he had complained about the navy and being a victim of discrimination and had several incidents with law enforcement, including two shootings
Alexis carried three weapons in the attack: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene.
Police said they now believed that he had acted alone. He had been a full-time navy reservist from 2007 to early 2011 and had a string of misconduct problems but received an honourable discharge.
Alexis had had shooting incidents in 2004 and 2010 in Fort Worth and Seattle.