A former Navy reservist who shot dead 12 people on a US naval yard before being killed in a shoot-out had been arrested twice previously for gun-related incidents, it has emerged.

In April 2010, Alexis was arrested for discharging a firearm in an apartment, according to a Fort Worth Police Department incident report.

He claimed the weapon went off accidentally when he was cleaning it.

Earlier in May 2004 he was arrested after shooting three rounds into the tires of a vehicle owned by construction workers, 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 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 was killed by armed police after launching an attack on the US Naval Sea Systems Command headquarters in Washington, DC, on Monday.

Authorities confirmed 12 victims, aged between 46 and 73, were killed during the shooting and another eight injured.

The scene outside the US Naval Sea Systems Command headquarters on Monday morning

The 34-year-old gunman used a valid pass to gain access to the Navy yard, where he worked as a contractor for The Experts which refreshed equipment on the Navy Marine Corps intranet network, the FBI revealed.

Speaking on Monday night, Washington DC Police Chief Cathy Lanier said inquiries had ruled out the possibility there was a second gunman during the attack.

She said: "We do feel comfortable we have the single and sole person responsible for the loss of life on the base today.

"We have now exhausted all means to either support or discount the report (of a second shooter)."

Earlier reports said officers were searching for another black male, aged between 40 and 50 and wearing an olive drab uniform.

The US Navy confirmed Alexis was a full-time Navy reservist between 2007 and 2011 when he was discharged as a petty officer third class for misconduct.

No further details of his discharge have been revealed.

The Washington Navy Yard, home to the US Naval Sea Systems Command headquarters, in 1985

Alexis was a Buddhist convert and worked as a waiter and delivery driver at a Thai restaurant in Fort Worth after his discharge.

Embry-Riddle Aeronautical University, which offers online courses in aviation and aerospace, confirmed that Alexis was enrolled as an online student via its Fort Worth campus, started classes in July 2012 and was pursuing a bachelor's of science in aeronautics.

He grew up in Brooklyn with his mother, Sarah, and father, Anthony Alexis, his aunt Helen Weeks told The Washington Post.

Witnesses described a gunman opening fire from the fourth floor of building 197 at the sprawling naval yard at around 8.20am on Monday as people arrived for work.

Yesterday evening, President Barack Obama paid tribute to the military staff who were killed or injured in the attack.