WASHINGTON (CNN) -- The picture emerging of the 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 morning 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.
Alexis used that ID to gain access to the Navy Yard, according to a U.S. law enforcement official with knowledge of the investigation.
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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.
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' family reeled at the news that he was believed to be the man behind the killings.
"What I do know is he wasn't that type of person," Anthony Little, who identified himself as Alexis' brother-in-law, told reporters outside his Brooklyn, New York, home. "I didn't really hear anything that would make me feel, as a newcomer to the family, that somebody should be watching him."
He said the family's initial reaction was "very distraught, very stressed out, tears."
"You know, they didn't see it coming," said Little, who is married to Alexis' sister Naomi. "Their hearts are going out more to the victims and the people that got hurt because, you know, there's more lives lost and we don't need that right now. We really don't."
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