Aaron Alexis seems a study in contradictions: a former US Navy reservist, a Defence Department contractor, a convert to Buddhism who was taking an online course in aeronautics. But he also had a temper that led to run-ins with police over shootings in Fort Worth, Texas, and in Seattle.
His motive remained a mystery, but US law enforcement officials said he suffered from paranoia, a sleep disorder, and was hearing voices in his head.
Family members said the 34-year-old had been under treatment since August by the federal Veterans Administration.
Some of his neighbours and acquaintances described him as nice and the navy had not declared him mentally unfit, which would have rescinded a security clearance he had from his earlier time in the Navy Reserves.
In Monday's attack, which left 13 people dead, he carried three weapons: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene.
The AR-15 is the same type of rifle used in last year's mass shooting at a Connecticut school that killed 20 students and six adults. The weapon was also used in the shooting at a Colorado cinema that killed 12 and wounded 70.
At the time of the shootings, Alexis worked for a subcontractor on an HP Enterprise Services contract to refresh equipment used on the Navy-Marine Corps intranet network.
He lived in Seattle in 2004 and 2005. In 2004, Seattle police arrested him for shooting out the tyres of another man's vehicle in what he described to detectives as an anger-fuelled "blackout".
Alexis stared at construction workers at a job site daily for several weeks prior to the shooting. The owner of the construction business told police he believed Alexis was angry over a parking issue at the site.
When police searched his home after his arrest they found a gun and ammunition in his room.
He told detectives he thought he was being mocked by construction workers the morning of the incident. He also claimed he could not remember firing his gun at the car. He was not prosecuted.
Alexis also told police he was present during "the tragic events of September 11, 2001" and described "how those events had disturbed him".
On May 5, 2007, he enlisted in the navy reserves, serving until 2011.
His last assignment was as an aviation electricians mate 3rd class at the Naval Air Station Joint Reserve Base in Fort Worth.
It was while he was still in the reserves that a neighbour in Fort Worth reported she had been nearly struck by a bullet shot from his downstairs apartment.
In September 2010, Fort Worth police questioned Alexis about the neighbour's report.
He admitted firing his weapon but said he was cleaning his gun when it accidentally discharged.
After leaving the reserves, he worked as a waiter and delivery driver at the Happy Bowl Thai restaurant in White Settlement, a suburb of Fort Worth. He also enrolled for online courses in aviation and aerospace in July last year.
Alexis knew some Thai, as he had travelled to Thailand, and could speak to Thai customers in their native language.
"We are all shocked. We are non-violent. Aaron was a very good practitioner of Buddhism. He could chant better than even some of the Thai congregants," said Ty Thairintr, a congregant at Wat Budsaya, a Buddhist temple in Fort Worth.
Thairintr said Alexis told him he was upset with the navy because "he thought he never got a promotion because of the colour of his skin. He hated his commander."
As Thairintr and others at the temple said Alexis took a job as a contractor and he indicated to them he was going to go to Virginia. They last saw him five weeks ago.
"He was a very devoted Buddhist. There was no tell-tale sign of this behaviour," Thairintr said.