(CBS News) Aaron Alexis, who was identified as the alleged gunman who died in Monday's deadly mass shooting at the Washington Navy Yard in 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," and in January 2011 Alexis received a general 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.
Police records also show Alexis had been involved in two minor shooting incidents before. 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.
Sources say Alexis was armed with three weapons: an assault rifle, a shotgun and a pistol. All of the guns were recovered near his body.
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.
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WASHINGTON (CBS News/AP) A former Navy reservist went on a shooting rampage Monday inside a building at the heavily secured Washington Navy Yard, firing from a balcony onto office workers in the cafeteria below, authorities and witnesses said. Twelve people were killed, plus the gunman.
The FBI took charge of the investigation and identified the gunman killed in the attack as 34-year-old Aaron Alexis of Texas. He died after a running gunbattle with police, investigators said.
Authorities said they were looking for a possible second attacker who may have been disguised in an olive-drab military-style uniform.
But as the day wore on and night fell, the rampage increasingly appeared to be the work of a lone gunman, and Navy Yard employees were gradually being released from the complex and children were let out of their locked-down schools.
"We are confronting yet another mass shooting," President Obama said Monday during remarks that were intended to address the financial crisis. "And today it happened on a military installation in our nation's capital."
A law enforcement official told CBS News' Pat Milton the shooter was discovered with an AR-15 assault rifle and a shotgun with him and was wearing dark blue clothing.
Milton reports that the shotgun was traced to a gun store in Lorton, Va., according to a federal law enforcement official. The official said the shotgun was purchased at the store within the last week.
CBS News correspondent Bob Orr reports that Alexis was also found with a sidearm that authorities believe may have come from one of the Navy Yard's security officers.
Investigators said they had not established a motive for the attack, which unfolded about 8:15 a.m. in the heart of the nation's capital, less than four miles from the White House and two miles from the Capitol.
As for whether it may have been a terrorist attack, Mayor Vincent Gray said: "We don't have any reason to think that at this stage." But he said the possibility had not been ruled out.
It was the deadliest shooting rampage at a U.S.-based military installation since Maj. Nidal Hasan killed 13 people and wounded more than 30 others in 2009 at Fort Hood in Texas. He was convicted last month and sentenced to death.
Mr. Obama promised to make sure "whoever carried out this cowardly act is held responsible."
On Monday, Alexis was carrying an ID card belonging to Rollie Chance, who was placed on administrative leave last October. Chance told officials he does not know Alexis.
At the time of the rampage, Alexis was working in information technology with a company that was a Defense Department subcontractor.
Alexis was a full-time Navy reservist from 2007 to early 2011, leaving as a petty officer third class, the Navy said. It did not say why he left. He had been an aviation electrician's mate with a unit in Fort Worth, Texas.
Patricia Ward, a logistics-management specialist, said she was in the cafeteria getting breakfast.
"It was three gunshots straight in a row — pop, pop, pop. Three seconds later, it was pop, pop, pop, pop, pop, so it was like about a total of seven gunshots, and we just started running," Ward said.
In addition to those killed, more than a dozen people were hurt, including a police officer and two female civilians who were shot and wounded. They were all expected to survive.
The Washington Navy Yard is a sprawling labyrinth of buildings and streets protected by armed guards and metal detectors, and employees have to show their IDs at doors and gates to come and go. About 20,000 people work there.
The rampage took place at Building 197, the headquarters for Naval Sea Systems Command, which buys, builds and maintains ships, submarines and combat systems. About 3,000 people work at headquarters, many of them civilians.
Witnesses described a gunman opening fire from a fourth-floor overlook, aiming down on people in the cafeteria on the main floor. Others said a gunman fired at them in a third-floor hallway.
Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other co-workers encountered a gunman in a long hallway on the third floor. The gunman was wearing all blue, he said.
"He just turned and started firing," Brundidge said.
Terrie Durham, an executive assistant with the same agency, said the gunman fired toward her and Brundidge.
"He aimed high and missed," she said. "He said nothing. As soon as I realized he was shooting, we just said, 'Get out of the building.'"
In the confusion, police said around midday that they were searching for two men who may have taken part in the attack — one carrying a handgun and wearing a tan Navy-style uniform and a beret, the other armed with a long gun and wearing an olive-green uniform. Washington Police Chief Cathy Lanier said it was unclear if the men were members of the military.
But later in the day, police said the man in the tan uniform had been identified and was not involved in the shooting.
As emergency vehicles and law enforcement officers flooded streets around the complex, a helicopter hovered, nearby schools were locked down and airplanes at nearby Reagan National Airport were grounded so they would not interfere with law-enforcement choppers.
Security was tightened at other federal buildings. Senate officials shut down their side of the Capitol while authorities searched for the potential second attacker. The House remained open.
Adm. Jonathan Greenert, chief of naval operations, was at the base at the time the shooting began but was moved unharmed to a nearby military installation.
Anxious relatives and friends of those who work at the complex waited to hear from loved ones.
Tech Sgt. David Reyes, who works at Andrews Air Force Base, said he was waiting to pick up his wife, Dina, who was under lockdown in a building next to where the shooting happened. She sent him a text message.
"They are under lockdown because they just don't know," Reyes said. "They have to check every building in there, and they have to check every room and just, of course, a lot of rooms and a lot of buildings."
According to public records, Alexis' neighbor called Fort Worth police in September 2010 after she was nearly struck by a bullet that came from his downstairs apartment. Alexis told police he was cleaning his gun when it went off.
He was arrested on suspicion of discharging a firearm within city limits but was not prosecuted.
EDITORS NOTE: CBS News incorrectly identified the suspected shooter in earlier reports. Read this piece for an explanation of the initial confusion.
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