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Crazed Aaron Alexis was treated for mental illness after playing violent video games for up to 18 hours day and night.

The 34-year-old told psychiatrists he heard voices in his head long before he went on the rampage at a US naval base and slaughtered 12 people before being shot dead himself.

Friends said the length of time he spent glued to the “shoot ‘em up” games on his computer, including the popular Call of Duty, triggered his dark side that had previously landed him in trouble with the police on gun crimes.

But despite his psychiatric problems and arrests for violence, the IT contractor was never declared mentally unfit by the US Navy – a move that would have stripped him of the security pass that allowed him to drive unchallenged into the ­Washington base and carry out his murderous mission.

And as a result, his gun licence was also never rescinded.

Friend Nutpisit Suthamtewakul said “hard-core drinker” former navy reservist Alexis became so hooked on video games he would have to take him food as he would not tear himself away to eat.

He added: “He could be in the game all day and all night. I think games might be what pushed him that way.”

Alexis had worked at Suthamtewakul’s Happy Bowl Thai restaurant in Fort Worth, Texas.

His best pal added: “He always had this fear people would steal his stuff so that’s why he would carry his gun all the time.

"He would carry it when he was helping out in the restaurant which scared my customers.”

Happy Bowl customer Chris Childs said he would often chat to Alexis about violent video games.

He said the killer was “really into” Call of Duty and World of Warcraft.

Mr Childs added: “There were a couple of guys I worked with that were kind of into shooter games, but by the time a new one came out, he would know a lot about the game.”

Another friend, Michael Ritrovato, told how he saw twisted Alexis playing the part of a killer in the games.

A source close to the case said: “What is emerging is a picture of someone who was clearly troubled but yet no one in authority picked up on it.

“There are several events in his life that should have set alarm bells ringing that he was unsuitable to be around firearms.

"We know he had an obsession with violent video games and we will look into that part of the investigation.”

Experts have argued for many years people addicted to violent computer games can be affected by their content.

Call Of Duty are first-person shooting games where the gamer plays the role of military men on realistic missions.

Alexis went to the Veterans Administration last month seeking treatment for ­paranoia, insomnia and possible schizophrenia.

Around the same time he called police to his hotel room in Rhode Island after complaining of hearing voices.

He had also claimed to be suffering from post traumatic stress disorder after helping with rescue efforts in the wake of the New York 9/11 attacks.

Troubling questions have already been asked of the US Department of Defense which cleared the killer for military base access twice in the past 12 months.

A Google search would flag up his previous arrests for gun offences and Congress is demanding to know how security staff failed to pick up on them.

Alexis had visited six military ­installations across America and Japan while working in IT for a Hewlett-Packard sub-contractor.

He was employed by a firm called The Experts. CEO Thomas Hosko confirmed Alexis had been passed by the ­Department of Defense.

He said: “He did have a secret clearance. We had recently rehired him.

"Another background investigation was rerun and cleared through the security service in July 2013.”

The pass would permit Alexis to drive on to the navy base, allowing him to hide his weapons from security.

Mr Hosko added further checks on the gunman “revealed no issues other than one minor traffic offence”.

Alexis, who served full-time in the navy between 2007 and 2011 before being discharged over misconduct, was shot dead by police after he opened fire inside the base around 8:20am on Monday.

He used three guns – a shotgun, 9mm pistol and an AR-15 semi-automatic rifle – the same type of weapon used in the Sandy Hook and Aurora massacres.

After shooting some of his victims he stripped them of their weapons before becoming involved in a gun fight with police.

Witnesses said he had taken refuge in a command HQ and started picking off people in a cafeteria from a fourth-floor window.

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It is understood the killer purchased a shotgun in Lorton, Virginia, within the past week as he stayed in Washington’s Residence Inn hotel while preparing to start work on the base.

Alexis had been arrested by Seattle and Texas police for firing a gun in the past.

In 2004 he shot out the tyres of a construction worker’s truck in an “anger-fuelled blackout” after he thought he had been mocked.

Six years later he fired a bullet through his apartment ceiling, narrowly missing his neighbour, who he had complained about for making too much noise. He claimed the gun went off accidentally.

Yesterday, Defence Secretary Chuck Hagel laid a wreath at the US Navy Memorial in remembrance of those who were murdered.

Nine of the 12 civilian workers killed had been identified by last night. They were aged between 46 and 73. The eldest victim was John Roger Johnson, who had four daughters.

The logistics analyst set off for work early that day to beat rush hour and his wife Judy said: “He always said, ‘Goodbye beautiful, I love you so much. You have a good day and God bless you’.”

Of the 14 injured doctors have said all are expected to make a good recovery.

They include a woman shot in the head and police officer Scott Williams, whose legs were blasted.

Budget cuts may have also been behind the lax security at the base. Excerpts from a soon-to-be-released government audit reveal how screening of the yard’s 3,000 staff has suffered.

Washington Mayor Vincent Gray last night said such cuts “put people at risk”.

The federal security assessment done over the past year highlighted how ­contractors such as Alexis were given too much access at navy yards across the US.