WASHINGTON (CNN) -- The picture emerging of a dead gunman in Monday's rampage at the Washington Navy Yard is a study in contrasts, one of a man who practiced languages and meditated and another of a cold-blooded killer.
The gunman was identified as 34-year-old Aaron Alexis, a former Navy reservist and a current military contractor, the Washington FBI Field Office told CNN. His identity was confirmed by fingerprints and a picture ID card, the FBI said.
Authorities have not released a possible motive in the shooting at the headquarters for Naval Sea Systems Command that left more than a dozen people, including the gunman, dead. But a friend said Alexis was locked in a dispute over money with the company that contracted him to work for the Navy.
By nightfall, authorities said they were "confident" that Alexis was the lone gunman, bringing to an end a day-long police search for a possible second suspect.
Alexis was carrying a military-contractor ID that matched his appearance, a D.C. Metropolitan Police official told CNN on condition of anonymity.
He drove onto the installation and parked before walking a short distance to Building 197. Once inside, according to the official, Alexis made his way to an overlook above the atrium and opened fire.
Alexis was armed with an AR-15 semiautomatic rifle, a .9mm handgun and another rifle, the official said. He was believed to have used the semiautomatic rifle during most of the attack, the official said.
Even as the FBI ruled out any other shooters in the rampage, Metropolitan Police were trying to track down at least one person to determine whether they had any involvement.
Authorities are looking for a black male, between 40 and 50, with a medium complexion and gray sideburns who was reportedly wearing an "olive drab-colored" military-style uniform.
While the FBI was urging anyone with information about Alexis to come forward, investigators were focusing on reported incidents, including police run-ins, that portray a man with increasingly violent tendencies.
There were no indications that Alexis had any ideological differences with the Navy or any disagreements with anyone at the Navy Yard, the U.S. law enforcement official said.
Alexis, who was from New York City, served as a full-time Navy reservist between 2007 and 2011, according to military records.
In the Navy, Alexis achieved the rank of aviation electrician's mate 3rd class, working on aircraft electrical systems, the records show.
Alexis was discharged after a "pattern of misconduct," a U.S. defense official, with knowledge of the investigation, told CNN on condition of anonymity. The official did not detail the misconduct.
Most recently, Alexis worked as an information technology contractor with the Navy, Secretary of the Navy Ray Mabus told CNN's Wolf Blitzer.
Alexis appeared to have had sporadic run-ins with the law, dating back to at least 2004 when he was arrested in Seattle on charges he shot out the tires of a man's truck in an anger-fueled "blackout," according to a Seattle Police Department report.
He told investigators he believed the man, a construction worker, was mocking him, but had no memory of shooting out the tires, the report said.
Investigators later spoke with Alexis' father, who told police that his son had anger management problems associated with post-traumatic stress disorder, which he suffered after working "as an active participant in rescue attempts" during the September 11, 2001, attacks in New York, the report said.
In 2010, Alexis was arrested by Fort Worth, Texas, police but never charged over an allegation that he fired a gun through the ceiling of his apartment. According to records, he told police he accidentally fired it while cleaning it.
His last known address was outside of Fort Worth, Texas, where he was roommates for three years with Nutpisit Suthamtewakul, who described Alexis as his best friend.
Alexis befriended Suthamtewakul four years ago after he emigrated from Thailand.
Alexis taught him about American culture, Suthamtewakul told CNN. Alexis, he says, was fluent in Thai and attended a Buddhist Temple.
When Suthamtewakul opened the Happy Bowl Thai Restaurant, Alexis would occasionally help out, waiting tables, he said.
The two were roommates until five months ago when Suthamtewakul got married and Alexis had to move out. Toward the end, Alexis spent a lot of time holed up in his room, keeping to himself, Suthamtewakul said.
On Monday, Suthamtewakul reeled at the news that Alexis was believed to be a gunman in the rampage at the Navy Yard.
"I can't believe he did this," he said. "He never showed any sign of violence."
But there were signs that Alexis was unhappy. He was very frustrated with the company who contracted him to work for the Navy, according to a friend.
Alexis claimed he wasn't paid properly by the company after returning from a months-long assignment to Japan last year, said Michael Ritrovato, another former roommate.
It was unclear whether the dispute was over salary or expenses. Alexis just felt the company owed him money and had not paid him, Ritrovato said.
It is not clear whether Alexis was still living in Fort Worth area at the time of the shooting.
Alexis was believed to have arrived in the Washington area last week, when he reportedly checked into a hotel, according to someone who met him at the hotel. The person, who declined to be identified, said Alexis indicated that he planned to be in the area for several weeks.
At the time of the shooting, Alexis was working for The Experts, a subcontractor of HP Enterprise Services that was contracted to "refresh equipment used on the Navy Marine Corps Intranet network," according to a statement released by the company.
Alexis, who had Department of Defense security clearance, worked from September 2012 through January refreshing computer systems in Japan, said Thomas E. Hoshko, the CEO of The Experts.
His security clearance was renewed in July to carry out the same type of contract work at the Navy Yard, Hoshko said.
CNN's Evan Perez reported from Washington and Ed Lavandera reported from Fort Worth. Chelsea J. Carter wrote this report from Atlanta. CNN's Brian Todd, Susan Candiotti, Barbara Starr, Jake Tapper, John King, Pamela Brown, Deborah Feyerick, Alan Silverleib, Dan Merica and Larry Shaugnessy contributed to this report.
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