Navy Yard Shooting Update: Suspect Aaron Alexis worked for several days with subcontractor at Navy Yard before shooting, report says

(CBS/AP) -  Aaron Alexis worked for several days as a computer technician at the Washington Navy Yard with a Defense Department subcontractor before police say he gunned down 12 in a Monday rampage, CBS News reports.

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Alexis was an employee with The Experts, a company that was a a Defense Department subcontractor on a Navy-Marine Corps computer project, authorities said. A Hewlett-Packard Company spokesman confirmed that Alexis worked for TheExperts, a subcontractor to HP, CBS affiliate WUSA9 reports.

The job of "The Experts" is to "refresh equipment used on the Navy Marine Corps Intranet network," the spokesman said in a statement.

CBS News correspondent John Miller said on CBS This Morning Alexis attended work every day last week wiring up cubicles for the IT refresh, and co-workers said they got along with him and he seemed normal.

Alexis had access to the Navy Yard as a defense contractor and used a valid pass, said Valerie Parlave, head of the FBI's field office in Washington. However, the owner of "The Experts" told The Washington Post he wouldn't have hired Alexis had he known about his troubled history.

Alexis was two previous arrests - one in 2010 for allegedly shooting into the ceiling of his Fort Worth apartment and another in Seattle in 2004 for allegedly shooting out a construction worker's tires.

The former Navy reservist said he had been hearing voices and had sought treatment for mental illness, CBS News reported. In his four-year career at the Navy, he had been cited at least eight times for misconduct, the Washington Post reported.

"Anything that suggests criminal problems or mental health issues, that would be a flag. We would not have hired him." Thomas Hoshko, chief executive officer of The Experts, told The Washington Post.

Alexis had worked for the company since July at several installations including Arlington, Va., Cherry Point, N.C., and Stafford, Va., the Washington post reported. He had worked at the Washington Navy Yard for for several days before the shooting at the Navy Yard, according to the report.

The motive for the mass shooting - the deadliest on a military installation in the U.S. since the attack at Fort Hood, Texas, in 2009 -remains a mystery, investigators said.

U.S. law enforcement officials told The Associated Press that there was no known connection to terrorism and that investigators have found no manifesto or other writings suggesting a political or religious motive.

Alexis had been suffering a host of serious mental problems, including paranoia and a sleep disorder, and had been hearing voices in his head, according to the officials, who spoke on condition of anonymity because the criminal investigation was still going on.

He had been treated since August by Veterans Affairs, the officials said.

The Navy had not declared him mentally unfit, which would have rescinded a security clearance Alexis had from his earlier time in the Navy Reserves.

The assault is likely to raise more questions about the adequacy of the background checks done on contract employees and others who are issued security clearances.

In the hours after the Navy Yard attack, a profile of Alexis began coming into focus.

A Buddhist convert who had also had flare-ups of rage, Alexis, a black man who grew up in New York City and whose last known address was in Fort Worth, Texas, complained about the Navy and being a victim of discrimination. In additions to his run-ins with the law over shootings in 2004 and 2010 in Texas and Seattle, and was ticketed for disorderly conduct after being thrown out of a metro Atlanta nightclub in 2008.

Alexis' bouts of insubordination, disorderly conduct and being absent from work without authorization prompted the Navy to grant him an early - but honorable - discharge in 2011 after nearly four years as a full-time reservist, authorities said. During his service, he repaired aircraft electrical systems at Fort Worth.

Law enforcement officials said Alexis used a shotgun and two handguns, but not an AR-15 assault rifle, as authorities previously reported. He bought the shotgun and took the handguns away from law officers at the scene, an official said.

For much of the day Monday, authorities said they were looking for a possible second attacker who may have been disguised in an olive-drab military-style uniform. But by late Monday night, they said they were convinced the shooting was the work of a lone gunman, and the lockdown around the area was eased.

"We do now feel comfortable that we have the single and sole person responsible for the loss of life inside the base today," Washington Police Chief Cathy Lanier said.

President Barack Obama lamented yet another mass shooting in the U.S. that he said took the lives of American "patriots." He promised to make sure "whoever carried out this cowardly act is held responsible."

Complete coverage of the Washington Navy Yard shooting on Crimesider