Handout photo of Aaron Alexis, who the FBI believe to be responsible for the shootings at the Washington Navy Yard in the Southeast area of Washington, DC - (Reuters)
Aaron Alexis killed 12 people in gun rampage at a Washington navy yard (Reuters)

Aaron Alexis, the suspected gunman who died after killing 12 people at a naval yard in Washington DC, had received treatment for serious mental illness including hearing voices in his head.

Alexis - who was employed by The Experts, a Hewlett-Packard subcontractor that was working on a US Navy intranet network - had been treated since August by the Veterans Administration for his mental problems that included paranoia and a sleep disorder, said AP.

The navy had not declared him mentally unfit.

Alexis, who expressed a keen interest in Buddhism and Thai culture, served as a navy reservist for four years. He was kicked out in 2011 because of a "pattern of misconduct".

After his discharge Alexis began working as a civilian contractor at the yard with security clearance.

The motive for the mass shooting - the deadliest on a military base since a jihadist gunman killed 13 people at Fort Hood, Texas in 2009 - are shrouded in mystery.

Alexis had been involved in two minor shooting incidents. In 2004, he was arrested in Seattle for shooting another man's car tyres. He was arrested again in 2010 for firing a weapon into the ceiling of his apartment. He was not prosecuted in either cases.

The 34-year-old IT contractor carried three weapons in the naval yard incident: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene.

Aaron Alexis Named as Washington Navy Yard Gunman

Washington US Navy Yard: Four Dead in Gun Rampage [FIRST PICTURE+VIDEO]