Updated at 12:00 p.m. ET Tuesday with clarification on the weapons found at the scene.
(CBS News) Aaron Alexis, who was identified as the alleged gunman who died in Monday'sin the nation's capital, joined the Navy Reserves in May 2007, one day before his 28th birthday.
He then spent four years working as an aviation electrician's mate, most recently at the naval air station in Fort Worth, Texas. Military records show "a pattern of misconduct," though in January 2011 Alexis received an honorable discharge.
After leaving the service, Alexis worked as a defense contractor, and sources say he spent some time working at the Navy Yard where the shootings played out.
Alexis maintained a secret clearance from his work with the Navy.
(Watch CBS News senior correspondent John Miller discuss the investigation)
Police records also show Alexisbefore. He was arrested in Seattle back in 2004 for shooting out another man's tires. He was arrested again in Texas in 2010 for firing a weapon into the ceiling of his apartment. He was not prosecuted in either of those cases.
Alexis also apparently was not a big presence on social networking sites, and sources say so far they cannot connect him in any way to any known threats or terror groups. But the motive for the shootings remains unknown.
Investigators believe he carried out the attack with a shotgun, and apparently acquired one, possibly two, handguns during the incident. Law enforcement sources said Monday that an assault rifle was found at the scene, but authorities said Tuesday they did not believe Alexis used an assault rifle in the attack.
He worked his way to the third and fourth floors of a building, which then looked down on an open atrium.
Sources say with that assault rifle, which is believed to be an AR-15, Alexis essentially was a sniper, able to easily target his victims from a very high vantage point. He fired a large number of shots in a relatively confined area.
Investigators are working to trace the weapons. They're also mapping the trajectory of the shots to better understand exactly how the assault played out.
The FBI is looking for help to learn more about Alexis.
They've put out a poster asking anyone with information to telephone 1-800-CALL-FBI.
The poster is emblematic of how little they have about Alexis' motive in the investigation.
If you look at the idea that they have run through all of their traps and databases and they see Alexis doesn't turn up in any significant investigations and isn't mentioned in any terrorism probes, then they're still stuck on what's the possible motive.