A shooting rampage in the heart of a US Navy complex in Washington DC has left at least 13 people dead.
One shooter was killed, but police were looking for one other possible gunman wearing a military-style uniform.
The White House and Senate were put in lockdown, schools closed and a Major League Baseball clash postponed following the attack.
President Barack Obama said he was mourning “yet another mass shooting” and called it a “cowardly act.”
Officials said at least three people were wounded in the rampage at the Washington Navy Yard, including a law enforcement officer.
Hospital officials said all three were expected to recover.
Federal law enforcement officials identified the shooter as Aaron Alexis, a 34-year-old Texas man believed to have a criminal record there and said he may have gotten into the Navy Yard by using someone else’s identification card. He was killed after opening fire on police officers.
The two officials spoke to The Associated Press on condition of anonymity because they were not authorised to speak publicly.
The shooting led to tightened security at the Capitol and White House nearby, including shutting down the Senate while a possible remaining shooter was sought.
DC Police Chief Cathy Lanier announced the death toll and said people were being told to stay in their homes and out of the area.
Lanier said there was no indication of a possible motive.
The shooter was a Navy employee whose work status had been changed earlier this year, said a federal government official who spoke only on condition of anonymity because he wasn’t authorised to speak on the record.
While that may suggest the motive was job related, authorities haven’t ruled out anything including terrorism, the official said.
Police said a man in a tan, military-style outfit who had been sought in connection with the shooting had been identified and was not a suspect or a person of interest in the slayings.
The DC Police Department said the man had been identified and is not believed to be a gunman or otherwise involved in the shootings.
Witnesses described a gunman firing down on a cafeteria from an upper floor and a gunman firing at people in a hallway on another floor. It wasn’t clear whether the witnesses were describing the same gunman.
About 3000 people work at the Naval Sea Systems Command headquarters, which builds, buys and maintains the Navy’s ships and submarines and combat systems.
Todd Brundidge said he and other co-workers encountered a gunman in a hallway on the third floor. The gunman was wearing all blue, he said.
“He just turned and started firing,” Brundidge said.
Terrie Durham added, “He aimed high and missed. He said nothing. As soon as I realised he was shooting, we just said, ’Get out of the building.”’
Rick Mason said a gunman was shooting from a fourth-floor overlook in the hallway outside his office.
He said the gunman was aiming down at people in the building’s cafeteria. Mason said he could hear the shots but could not see a gunman.
Mason said there are multiple levels of security to reach his office. That “makes me think it might have been someone who works here,” he said.
One person died at George Washington University Hospital of a single gunshot wound to the left temple, said Dr Babak Sarani, director of trauma and acute care surgery.
Janis Orlowski, chief operating officer of Washington Hospital Center, told reporters the hospital was treating three gunshot victims in critical condition. One was a Washington police officer and two were civilian women.
Orlowski said the police officer had gunshot wounds to the legs. One woman had a gunshot wound to the shoulder, and the other had gunshot wounds to the head and hand.
The shooting sparked a massive show of force as police and federal agents surrounded the Navy Yard, cordoning off streets only blocks from the US Capitol, home of Congress.
US officials gave no indication of any link to terrorism while police said the motive for the attack on the naval installation was unknown.
The White House was locked down in a sudden security alert after a man set off firecrackers outside the gates, just hours after the massacre.
Secret Service spokesman Edwin Donovan confirmed the man set off firecrackers, discounting some earlier reports that shots were fired, in a incident that scrambled security teams at the White House.