Aaron Alexis, 34, was killed in a gun battle with police following his rampage at the US Naval Sea Systems Command headquarters in Washington DC on Monday morning.
Alexis, from Forth Worth, Texas, was arrested over gun-related incidents in 2004 and 2010 in Fort Worth and Seattle and was described in police reports as “seething with anger”.
The Washington DC police report also said Alexis – who was discharged from the Navy in 2011 – had been traumatised by being present at the 9/11 attacks.
Despite his run-ins with the law, Alexis had access to the highly secure Navy Yard as a defence contractor for IT giant Hewlett-Packard and used a valid pass to enter the base to carry out the shooting, the FBI said.
The Washington Navy Yard is a sprawling labyrinth of buildings and streets protected by armed guards and metal detectors, and employees have to show their IDs at doors and gates.
But Alexis was carrying three weapons – an AR-15 assault rifle, a shotgun and a handgun that he took from a police officer at the scene, officials said.
David Stevens, a Navy contractor who was on the third floor of Building 197 when the shooting began, told The Washington Post: “It’s unbelievable that someone could get a rifle in there.”
Police have named seven of his 12 victims as Michael Arnold, 59; Sylvia Fraser, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Kenneth Bernard Proctor, 46, and Vishnu Pandit, 61.
Another eight people – including three who were shot – were also injured in the shooting.
Seattle police said in May 2004 Alexis – a convert to Buddhism who grew up in New York City – 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 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 11, 2001.”
In April 2010, Alexis was arrested for shooting a bullet through his apartment ceiling in Fort Worth. He claimed the weapon went off accidentally when he was cleaning it.
Alexis had been a full-time Navy reservist from 2007 to early 2011, leaving as a petty officer third class, the Navy said.
The Navy did not say why he left, but his stint in the Navy was reportedly troubled.
Alexis received two medals – both are issued to large numbers of service members who served since the 9/11 attacks.
At the time of the rampage, Alexis was employed as an IT subcontractor for a company called The Experts, which was working on a Hewlett-Packard contract to upgrade equipment for an intranet network used by the US Marine Corps and Navy, HP said in a statement.
It was unclear if the military or HP had been aware of Alexis’ brushes with the law before he was hired for the IT job.
Mayor Vincent Gray said the motive of the shooter – who appears to have acted alone – was not known.
But Mr Gray said there was no indication it was a terrorist attack, but that possibility had not been ruled out.
“This is a horrific tragedy,” he said.
The shooting sparked a massive show of force as police and federal agents descended on the Navy Yard, less than a couple of miles from the Capitol.
Outside the Alexis family home in Brooklyn, Alexis’s brother-in-law Anthony Little said his relatives were distraught.
“Their hearts are going out more to the victims who got hurt because you know it’s more lives lost and we don’t need that right now,” Mr Little said.
US President Barack Obama lamented “yet another mass shooting” in America and paid tribute to the rapid response of emergency personnel on the scene.
In the confusion, police said around midday on Monday that they were searching for two accomplices who may have taken part in the attack, but they later confirmed they were not seeking any other gunmen.
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