The man suspected of shooting and killing 12 people at a US naval base in Washington on Monday had been working as a defense IT subcontractor for computer giant Hewlett Packard, company officials said.

Aaron Alexis, a former naval reservist, had been employed by a firm working on an HP contract to upgrade equipment used by the US Navy and Marine Corps, HP spokesman Michael Thacker said in an email.

"Aaron Alexis was an employee of a company called 'The Experts,' a subcontractor to an HP Enterprise Services contract to refresh equipment used on the Navy Marine Corps Intranet (NMCI) network," he said.

The California-based company said it was "deeply saddened" by the shooting rampage at the Navy Yard, in which Alexis also lost his life after trading gunfire with police.

"HP is cooperating fully with law enforcement as requested," Thacker said.

It was unclear if the Defense Department or HP had been aware of Alexis' brushes with the law, which reportedly included two shooting incidents, before he was hired for the IT work.

Military officers said Alexis had a pattern of "misconduct" during his four-year service as a naval reservist.