WASHINGTON - On the surface, Aaron Alexis' four-year stint in the Navy appears to have been routine.
But as word spread Monday afternoon that Alexis is suspected of gunning down 13 people at the Washington Navy Yard here, Navy officials who asked not to be identified said the 34-year-old veteran had a pattern of misconduct.
What may have caused Alexis to go on a shooting spree Monday morning remained a mystery to FBI and Navy officials, as well as acquaintances of the quiet man who split his most recent years between New York and Texas.
PHOTOS: U.S. Navy Yard shooting
Valerie Parlave, chief of the FBI's D.C. field division, appealed for the public's help to assist in providing information about the shooter's actions and movements prior to the attack. The FBI posted his photographs on its website.
Navy Secretary Ray Mabus told CNN that Alexis was serving as an information technology contractor at the time of the shooting. He had enlisted in the Navy in 2007 and rose to the rank of Aviation Electrician's Mate 3rd Class.
From 2008 to 2011, Alexis served with the Fleet Logistics Support Squadron in Fort Worth. He lived in that area and was arrested at least once in 2010 for firing a gun through the ceiling of his apartment. He told police it had been an accident.
Alexis lived most recently in New York City, the Navy said. He had relatives in Georgia and Seattle, Wash., according to public reports. He last voted in Queens, N.Y., in 2000.
The Forth Worth Star-Telegram located a self-described "best friend" of Alexis on Monday who expressed surprise at the news and said Alexis had been working for a computer contractor.
"He lived with me three years," Nutpisit Suthamtewakul, owner of Happy Bowl Thai, told the newspaper. "I don't think he'd do this. He has a gun. but I don't think he's that stupid. He didn't seem aggressive to me."
A short LinkedIn profile of Alexis said he attended Embry-Riddle Aeronautical University and worked as a network technician at SinglePoint Technologies.
Contributing: Rick Jervis in Austin, Texas; Jim Michaels, Tom Vanden Brook, Kevin Johnson and Peter Eisler in Washington, D.C.