The man who shot dead 12 people during a rampage at Washington DC's Navy Yard before being killed himself has been identified as a 34-year-old former Navy reservist, authorities say.
A total of 13 people were killed in the shooting, which took place in the military complex that is barely two kilometres from the Capitol Building - the centre of US government.
Officials say the suspected shooter was Aaron Alexis, a Texan who served in the US Navy Reserve from May 2007 to January 2011. He is believed to have a criminal record there and to be a holder of a concealed carry weapon permit.
"We have no indication of motive at this time," said Washington D.C. police chief Cathy Lanier.
Witnesses recounted a day of chaos and fear as the gunman roamed the Navy Yard. Tim Jirus, a US Navy commander, said he was speaking with a civilian in an alleyway about what was unfolding when a shot rang out. He looked in the direction of the noise and then looked down. The man he was talking with had been shot in the head.
"I was running for my life," said Jirus, who wasn't sure if the shooter was on the roof or behind him. "I was just lucky. There were two shots, he got that guy. He didn't get me."
At least four people were wounded, including a Washington police officer who responded to the incident. "We've obviously had a horrific tragedy," DC Mayor Vincent Gray said.
Alexis is believed to have got into the highly secure Navy Yard by using someone else's identification card, one official says. It is not yet clear if that individual was an accomplice or if that person's ID card was stolen.
Police had been searching for at least one further gunman who they believe may have been disguised in a military-style uniform, however reports say that the shooting was the act of one man.
The FBI has taken charge of the investigation.
At the White House, President Barack Obama mourned what he yet another mass shooting in the US that he said took the lives of American patriots. Obama promised to make sure "whoever carried out this cowardly act is held responsible."
The area that was targeted, known as Building 197, was part of the headquarters for Naval Sea Systems Command, which buys, builds and maintains ships, submarines and combat systems. About 3000 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.
Around midday, police said they were searching for two men who may have taken part in the attack - one carrying a handgun and wearing a tan Navy-style uniform and a beret, the other armed with a long gun and wearing an olive-green uniform.
But later in the day, police said in a tweet that the man in the tan uniform had been identified and was not involved in the shooting.
As emergency vehicles and law enforcement officers flooded streets around the complex, a helicopter hovered overhead, nearby schools were locked down and airplanes at nearby Reagan National Airport were grounded so they would not interfere with law-enforcement choppers.
A short distance away, security was beefed up at the Capitol and other federal buildings, but officials said there was no known threat.
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," Brundidge said.
Terrie Durham, an executive assistant with the same agency, said she also saw the gunman firing toward her and 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 programme management analyst for the Navy who works on the fourth floor of the building, said a gunman was firing from the overlook in the hallway outside his office.
Shortly after the gunfire, 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 said.
Ward said security officers started directing people out of the building with guns drawn.
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 Center, said Janis Orlowski, the hospital's chief operating officer.
Orlowski said the police officer was in the operating room with gunshot wounds to the legs. The police chief said the officer was wounded when he engaged the shooter who later died.
One woman at the hospital had a gunshot wound to the shoulder. The other had gunshot wounds to the head and hand.
Admiral Jonathan Greenert, chief of naval operations, was at the base at the time the shooting began but was moved unharmed to a nearby military installation
Anxious relatives and friends of those who work at the complex waited to hear from loved ones.
Technical Sergeant David Reyes, who works at Andrews Air Force Base, said he was waiting to pick up his wife, Dina, who was under lockdown in a building next to where the shooting happened. She sent him a text message about being on lockdown.
"They are under lockdown because they just don't know," Reyes said. "They have to check every building in there, and they have to check every room and just, of course, a lot of rooms and a lot of buildings."
Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget. Only security personnel were allowed to be armed on the grounds.
Mason said there are multiple levels of security to reach his office. Everyone must show a building ID to get through a main gate, and at the building entrance, everyone must swipe a badge to pass through either a door or gate, depending on the entrance.
That "makes me think it might have been someone who works here," he said.
The Navy Yard has three gates, according to its website. One is open around the clock and must be used by visitors. A second gate is only for military and civilian Defense Department employees. The third gate is for bus traffic.
The Navy Yard is part of a fast-growing neighbourhood on the banks of the Anacostia River in southeast Washington, blocks from the Nationals Park baseball stadium.
- Reuters, AP, Washington Post
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