Profile: Navy Yard shooter Aaron Alexis

Aaron Alexis, the Navy contractor who killed 12 people at the Washington Navy Yard in the US capital on 16 September, believed he was being controlled by electromagnetic waves, according to the FBI.

The military veteran and Buddhist convert had a history of violent outbursts and paranoia, and etchings on the shotgun Alexis used in the attack shed light on his mental state.

On Wednesday, Valerie Parlave, head of the FBI's field office in Washington, said an electronic document belonging to the gunman indicated he had held delusional beliefs for months prior to the shooting.

"Ultra low frequency attack is what I've been subject to for the last 3 months, and to be perfectly honest that is what has driven me to this," Alexis, 34, reportedly wrote.

Alexis also etched phrases, including "End to the torment", "Not what y'all say", and "Better off this way" into the sawed-off Remington 870 shotgun he wielded in the attack.

Ms Parlave said the FBI investigation indicated Alexis had not targeted particular people at the Navy Yard, and that he was prepared to die and "accepted death as the inevitable consequence of his actions".

'Insubordination'

These are the latest details to emerge about Alexis' troubled past.

Little is known about New York-born Alexis' life before 2004, when he was arrested in Seattle for shooting the tyres of a construction worker's vehicle - in what he later described as an enraged "blackout".

His father told Seattle detectives after the incident that his son had been involved in rescue efforts during the 9/11 attacks in New York, and suffered from post-traumatic stress disorder as a result.

In August 2008, he was arrested for disorderly conduct in the state of Georgia, according to the Smoking Gun, a website that collects police and court documents.

In 2010, police were called to Alexis' apartment in Fort Worth, after an upstairs neighbour reported he had almost shot her through the floor. He told police his gun accidentally discharged while he was cleaning it, and no charges were filed.

Defence officials said Alexis served as a full-time reservist in the Navy from 2007-11, attaining the rank of petty officer 3rd class.

Posted at Fort Worth, Texas, he received two routine awards, the National Defense Service Medal and the Global War on Terrorism Service Medal.

'Nice and genteel'

After his discharge Alexis began pursuing a bachelor's degree in aeronautics through online classes and worked at a Thai restaurant in Fort Worth, where he was popular among the customers.

Friends and former colleagues of Alexis in Fort Worth expressed surprise on Monday

A former Fort Worth roommate, who described Alexis as his "best friend", said he was shocked by the news.

"I don't think he'd do this," Nutpisit Suthamtewakul, owner of Happy Bowl Thai restaurant, where Alexis had worked, told the Fort Worth Star-Telegram newspaper.

"He has a gun, but I don't think he's that stupid," Mr Suthamtewakul said.

Bud Kennedy, a columnist for the newspaper, told the BBC: "He was a quiet fellow and always nice and genteel."

Alexis attended a Buddhist temple in a suburb of Fort Worth, and had reportedly converted to the faith.

J Sirun, an assistant to the monks at the temple, told the Washington Post: "He did not like to be close with anybody, like a soldier who has been at war."

Hotel 'voices'

Alexis had been helpful to others at the temple, Mr Sirun told the newspaper, but at least one person there avoided him because he seemed too tightly wound.

At the time of the shooting, he was employed by a subcontractor working on a US Navy intranet network and had a valid pass to enter Washington Navy Yard.

A month before the shootings, while working as a contractor in Newport, Rhode Island, Alexis called police to his hotel room, while suffering an apparent paranoid episode.

He told officers someone he had argued with while boarding a plane in Virginia had sent three people to follow him.

Alexis believed they were using a microwave machine to send vibrations into his body so he could not sleep.

He checked into several hotels in order to evade them, according to media reports.