Aaron Alexis's motive in the navy yard rampage so far remains a mystery. However, law enforcement officials said he had paranoia and a sleep disorder and was hearing voices in his head.
They also said there had been no connection to international or domestic terrorism and investigators had found no manifesto or other writings to suggest a political or religious motivation.
The attack was unlikely to lead to tighter gun controls, political analysts said.
Measures proposed during national outrage over a school shooting in December that killed 20 children failed this year in Congress.
It was at least the seventh mass shooting of Barack Obama's presidency.
Relatvies said 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The Navy had not declared its defence contract employee mentally unfit, which would have rescinded the security clearance that Alexis had from his earlier time in the Navy Reserves.
Alexis used a valid pass to get into the Washington Navy Yard. In the past, he had complained about the Navy and being a victim of discrimination and had been involved in several incidents with police, including two shootings.
Alexis carried three weapons in the attack: an AR-15 assault rifle, a shotgun, and a handgun he took from a police officer at the scene. Police said they were convinced Alexis had acted alone.
At the time of the shooting, he was an employee with a company that was a Defence Department subcontractor on a Navy-Marine Corps computer project.
His employer said he had worked at six military bases over the summer without incident and had only started working at the navy yard, where 3000 people are employed, last week.
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.
A convert to Buddhism who grew up in New York City, Alexis had been involved in shooting incidents in 2004 and 2010 in Fort Worth and Seattle and was portrayed in police reports as seething with anger.
US Navy Secretary Ray Mabus yesterday ordered a review of physical security at all Navy and Marine Corps installations in the wake of Monday's shooting rampage.
Officials said that the Navy would first take a "quick look" at installations to make ensure existing physical security standards are being met while a second review would be larger and more in-depth.
A US congressman said he understood an earlier year-long investigation of security at the country's Navy installations had shown serious flaws partly driven by cost-cutting.
Representative Mike Turner, the chairman of a House Armed Services subcommittee, asked the Defence Department's inspector general's office to provide information about the audit of Naval security, which has yet to be published.
"Given the disturbing events (on Monday), I am highly concerned the access control systems at our nation's military installations have serious security flaws," Mr Turner wrote in a letter to Acting Inspector General Lynn Halbrooks.
The Ohio Republican added: "It is my understanding the report indicates the Navy may have implemented an unproven system in order to cut costs."