United States authorities are struggling to work out why gunman Aaron Alexis took his rifle to work at high security naval base in Washington D.C. and shot dead a dozen people.
Mr Alexis, aged 34, of Fort Worth, Texas, was the former serviceman who went on a rampage at Washington Navy Yard.
He had been working as a subcontractor for the computer company Hewlett Packard and had a legitimate pass to be at the base.
He took a legally-purchased rifle onto the base, and shot down into an atrium containing a food court.
The Washington Navy Yard is the headquarters of the Navy Sea Systems Command.
Senate officials made the decision to temporarily seal buildings on the US Capitol grounds about 3km from the Navy Yard.
Dozens of police and emergency vehicles surrounded the complex.
Mr Alexis, 34, had been working as a IT subcontractor for the computer giant Hewlett Packard, AFP reports.
Military officers say he had a pattern of misconduct during his four-year service as a naval reservist, and a former FBI officer told CNN that Mr Alexis was reported to have been unhappy about his discharge from the naval reserve.
He was also reported to have had anger-management problems.
In 2004 Alexis was arrested in Seattle after shooting out a man's tires in what he described as a rage-fueled "blackout".
And in 2010 Mr Alexis was arrested in Fort Worth, Texas for firing a pistol through his ceiling to silence an allegedly noisy neighbour.
It is still not clear just what motive he might have had for the latest shootings but police say there is no reason to suspect the rampage was a terrorist act.
Washington mayor Vince Grey says the shooting is a horrific tragedy, one of the worst in recent memory.
The shooting began as staff were arriving for the day at the yard, where about 3000 people work, and many have described a frantic scramble as a gunman took aim.
Witness Todd Grundig says he saw a gunman as he was leaving the building. "As we were exiting the back door we noticed him down the hall. He stepped around the corner, we heard shots, and as he came around the corner he aimed his gun at us and he fired at least two or three shots," he says.
Terri Durham says she was working at her desk when fire alarms went off and wardens began yelling for people to leave the building. She also saw a gunman as she fled.
"He was far enough down the hall that we couldn't see his face but we could see him with the rifle and he raised and aimed at us and fired, and he hit high on the wall just as we were trying to leave," she says.