At least 12 people have died in a shooting rampage at a US navy yard in Washington DC, police said.
One gunman had been killed but police warned there were possibly two others still on the loose.
One officer was shot in a gunfight with one of the men.
Both the remaining suspects, one black, one white, were wearing military-style uniforms although they were not believed to be serving troops.
Police Chief Cathy Lanier confirmed the death toll during a news conference.
She said people were being told to stay in their homes and out of the area as authorities search for the suspects.
There was no indication of a possible motive, she added.
The shootings happened inside one of the Navy's oldest buildings, trageting office workers at a heavily guarded military facility in the heart of the nation's capital.
The attack unfolded just a short distance from the White House and the US Capitol at a former shipyard that is one of the Navy's oldest shore facilities.
President Barack Obama mourned yet another mass shooting in the U.S. that he said took the lives of American patriots. He promised to make sure "whoever carried out this cowardly act is held responsible."
The building that was targeted was the military's headquarters for Naval Sea Systems Command, which buys, builds and maintains ships, submarines and combat systems. About 3,000 people work at the headquarters, many of them civilians.
Witnesses described a gunman opening fire from a fourth-floor overlook, aiming down on people in the first-floor cafeteria. Others said a gunman fired at them in a third-floor hallway.
Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other co-workers encountered a gunman in a long hallway on the third floor. The gunman was wearing all blue, he said.
"He just turned and started firing," Mr Brundidge said.
Terrie Durham, an executive assistant with the same agency, said she also saw the gunman firing toward her and Mr Brundidge.
"He aimed high and missed," she said. "He said nothing. As soon as I realised he was shooting, we just said, 'Get out of the building."'
Rick Mason, a civilian program-management analyst for the Navy, said a gunman was shooting from the overlook in the hallway outside his office.
Shortly after the gunfire, Mr Mason said, someone on an overhead speaker told workers to seek shelter and later to head for the gates at the complex.
Patricia Ward, a logistics-management specialist, said she was in the cafeteria getting breakfast.
"It was three gunshots straight in a row - pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop, so it was like about a total of seven gunshots, and we just started running," Ward told reporters several blocks away from the Navy Yard.
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. A police officer and two civilian women were in critical condition at Washington Hospital Centre, said Janis Orlowski, the hospital's chief operating officer.
Ms Orlowski said the police officer was in the operating room with gunshot wounds to the legs. One woman had a gunshot wound to the shoulder. The other had gunshot wounds to the head and hand.