A gunman who died during the shootout which killed at least 12 people in a US naval facility has been named as Aaron Alexis.

Alexis, who was 34 and from Fort Worth, Texas, was identified by fingerprints after being killed during the fight at the Washington Navy Yard.

Sources in Washington suggest the gunman was a civilian contractor, who used a fake ID to gain access to the installation. Police believe he may have been acting in concert with another shooter, and authorities say they are currently searching for a black male aged between 40 and 50 years old.

Little is known about Alexis's background, although police records show he was arrested in 2010 for discharging a firearm.

A motive for the attack has yet to be established, while a LinkedIn profile which listed an ex-navy employee called Aaron Alexis has now been removed.

Washington Mayor Mayor Vincent Gray has said he and his fellow officials "don't have any reason to think" the rampage was a terrorist act at this stage.

The FBI has taken control of the crime scene, and the White House, which lies just four miles from the navy yard, has been placed under lockdown, although officials insist there is no known threat.

President Barack Obama has promised swift action, saying it is crucial that "whoever carried out this cowardly act is held responsible."