The suspected gunman in Monday's rampage at the Washington Navy Yard is being described as both a polite, practising Buddhist, as well as a man prone to insecurity, rage and paranoia.

The FBI has identified Aaron Alexis, a 34-year-old former Navy reservist and a current military contractor, as the lone gunman in the rampage that left 12 people dead and several injured.

The motive for the shooting remains unclear, but there are reports Alexis may have been suffering from mental health problems. U.S. law enforcement officials tell The Associated Press that Alexis had recently sought help from the U.S. Veterans Administration for paranoia and delusions, as well as a sleep disorder. Family members told investigators that Alexis was under treatment.

Born in 1979, Alexis grew up in New York City. He was reportedly badly shaken by the Sept. 11, 2001 attacks on the World Trade Center and moved to Seattle in the early 2000s.

In 2004, Alexis had his first major brush with the law, when he was arrested at his Seattle home for malicious mischief.

Workers at a construction site next to Alexis' home alleged he had stared at them at the job site daily for several weeks before eventually shooting out the tires of one of their vehicles. The owner of the construction business told police he believed Alexis was angry over the parking situation around the site as well as the vehicle's owner, who had reportedly mocked him earlier that day.

Alexis told police he could not remember firing his gun, and said the incident happened during an anger-fueled "blackout." Charges were never filed.

Alexis' father told police after the incident that his son had been dealing with anger management problems and post-traumatic stress disorder, suffered after Alexis worked "in rescue attempts" following the 9/11 attacks.

In May, 2007, Alexis moved to Fort Worth, Texas where he enlisted in the Navy reserves. He served through 2011, never rising beyond the rank of petty officer, 3rd class. He received the National Defence Service Medal and the Global War on Terrorism Service Medal, two commonly-awarded medals.

In September 2010, while still serving in the reserves, he was arrested by Fort Worth police after a neighbour reported she had been nearly hit by a bullet shot from Alexis' apartment. The neighbour told police she believd Alexis fired intentionally because he had complained about her making too much noise.

Alexis admitted to firing his weapon, but said he was cleaning his gun when it discharged. Although he was arrested on suspicion of discharging a firearm, the case was not pursued after it was determined the gun discharged accidentally.

One Navy official told The Washington Post that Alexis was honourably discharged from the reserves shortly afterwards for “a pattern of misconduct,” which included the 2010 gun incident.

Upon leaving the reserves, Alexis worked as a waiter at a Thai restaurant in a suburb of Fort Worth after befriending the restaurant's owner, Oui Suthamtewakul. Alexis had spent a month in Thailand and had learned to speak to Thai customers in their native language. He had also converted to Buddhism and prayed at a local Buddhist temple.

He also enrolled in July 2012 as an online student at Embry-Riddle Aeronautical University, and was pursuing a bachelor's of science in aeronautics.

About a month ago, Alexis moved from Fort Worth to Washington to work as a tech employee for The Experts, a Hewlett-Packard subcontractor updating computer systems at Navy and Marine Corps installations. He was scheduled to begin work at the Washington Navy Yard this month.

Suthamtewakul told several media outlets Monday that Alexis had always seemed angry with the Navy and believed that racial discrimination prevented him from being promoted in the ranks.

Suthamtewakul told The Washington Post Alexis carried a gun on him at all times and also drank frequently, often starting at 9:30 in the morning. He described Alexis as an insecure though chatty man who had to reprimanded for awkwardly trying to hit on female customers.

“He’s a 13-year-old stuck in a 34-year-old body,” Suthamtewakul told the Post of Alexis. “He needs attention.”

With files from The Associated Press