A former Navy reservist who gunned down 12 people in an attack on one of the US's biggest navy yards had previously carried out an anger-fuelled shooting and was traumatised by 9/11, it has emerged.
Aaron Alexis, from Forth Worth, Texas, was killed in a shoot-out with police after launching an early morning attack on the US Naval Sea Systems Command headquarters.
It comes after the White House was placed under lockdown following a security alert on Tuesday evening. The Secret Service later said a man threw firecrackers over the north fence line before he was arrested.
The US Navy confirmed Alexis, a military contractor, was a full-time Navy reservist between 2007 and 2011 when he was discharged as a petty officer third class for misconduct.
He was working for the fleet logistics support squadron No 46 in Forth Worth, Texas, when he left the service in January 2011. No further details of his discharge have been revealed.
Seattle Police Department have released details of an incident in May 2004 in which Alexis shot out the tyres of a car claiming he had been traumatised by the 9/11 attacks.
According to the report Alexis claimed men on a construction site had been mocking him and he had suffered an "anger-fuelled blackout".
The report released by the police department said: "Alexis also told police he was present during 'the tragic events of September 11, 2001' and described 'how those events had disturbed him'.
"Detectives later spoke with Alexis' father, who lived in New York at the time, who told police Alexis had anger management problems associated with Post-Traumatic Stress Disorder, and that Alexis had been an active participant in rescue attempts on September 11th, 2001."
Alexis grew up in Brooklyn with his mother, Sarah, and father, Anthony Alexis, his aunt Helen Weeks told the Washington Post.
She said: "We haven't seen him for years. I know he was in the military. He served abroad. I think he was doing some kind of computer work."
Despite the shooting incident he became a Navy reservist, however, a Forth Worth Police Department incident report from April 2010 shows Alexis was arrested for discharging a firearm in an apartment.
According to the report, seen by Sky News, Alexis told police it went off accidentally when he was cleaning it.
He was awarded the Global War on Terrorism Service Medal and the National Defense Service Medal prior to his discharge
The FBI launched an appeal for further information about Alexis as police officers confirmed they were hunting a second man, dressed in military uniform, who has been described as a "potential gunman".
The Senate building and ten schools have been shut down as security precautions and residents have been advised to remain in their homes as the operation continues.
The attack on building 197 at the sprawling naval yard came at around 8.20am as people arrived for work at the command centre, which houses 3,000 military and civilian naval personnel.
Hundreds of police officers and airborne SWAT teams were drafted in within minutes of the first emergency calls and exchanged fire with Alexis.
A police officer was wounded in the firefight and was being treated for critical injuries in hospital.
It has been suggested Alexis may have used the identification card of another person to gain entry to the complex.
Police confirmed that 13 people, including Alexis, died in the incident but said that no motive had been established for what President Barack Obama described as "yet another mass shooting".
Washington DC Police Chief Cathy Lanier said that police were still hunting a black man, between 40 and 50, and dressed in olive military uniform. He was holding a "long gun".
Police said they had ruled out a white man dressed in a tan military uniform that they had previously described as a "potential gunman".
They said he was no longer a suspect or a "person of interest".
Washington DC Mayor Vincent C Gray said that there was no reason to suspect the incident was a "terrorist" attack but that it had not been ruled out.
Witnesses earlier described a gunman opening fire from the fourth floor, aiming down on people in the first-floor cafeteria. Others said a gunman fired at them in a third-floor hallway.
One employee at the Navy Yard, Patrica Ward, said she had just paid for her breakfast at a cafeteria when shots rang out.
"I was waiting for my friend to pay for when we heard the gun shot. It was three gun shots straight in a row, 'pow-pow-pow,'" she said.
"Three seconds later it was 'pow-pow-pow.' So it was like a total of seven gun shots. And we just started running."
The guard "told all of us to just run, to get away as fast as you can."
Responding to questions about the level of security at the base Ms Ward said: "It's not secure enough for me."
Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other co-workers encountered a gunman in a hallway of their building on the third floor.
"He just turned and started firing," he said.
Terrie Durham, an executive assistant, said she also saw the gunman firing toward her and Mr Brundridge.
"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.'"
One man in his 60s was taken to George Washington University Hospital with a gunshot wound to his head but died shortly after arriving. Dr Babak Sarani, director of trauma and acute surgery, said his injuries were so severe they could not be survived.
Janis Orlowski, the chief medical officer at Washington Hospital Center, said three victims had been taken to the hospital and while they were in a critical condition they were alert and able to speak. Two were women civilians.
She said: "They haven’t been able to give us any information about what happened to them. Obviously, they’re in pain and distress."
She said she thought the weapon must have been a semi-automatic "because they’re talking about gunshots they heard in rapid succession."
Officials at MedStar Washington Hospital Center said two shooting victims had been brought there.
Speaking ahead of a planned speech on the economy, President Barack Obama, who was being regularly briefed on the rapidly unfolding situation, said the attack was a "tragedy" and confirmed a number of people had been killed.
He said: "We are confronting yet another mass shooting and today it happened on a military institution in our nation's capital.
"It's a shooting that targeted our military and civilian personnel. These are men and women who were going to work doing their job protecting all of us.
"They are patriots and they know the dangers of serving abroad but today they faced the unimaginable violence that they wouldn't expect here at home."
He later thanked the FBI for its quick response to the fatal shooting.
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.
It builds, buys and maintains the Navy's ships and submarines and their combat systems.