By Ian Simpson
WASHINGTON, Sept 18 (Reuters) - The Washington Navy Yard remained closed to all but essential personnel on Wednesday as investigators tried to find out what triggered a shooting rampage by a contractor with a history of mental health problems.
The gunman, former Navy reservist Aaron Alexis, killed 12 people at the complex a mile and a half (2.5 km) from the U.S. Capitol and three miles (5 km) from the White House on Monday before police shot him dead.
The incident sent shockwaves through Washington and prompted the Pentagon to order a review of security practices at installations around the world.
Alexis, a 34-year-old military contractor with secret security clearance, had gained access to the base with a valid pass, carrying a legally purchased shotgun that he used in his attack. He also gained access to a handgun after beginning his assault, officials said.
Rhode Island police had warned the U.S. Navy last month that Alexis had called them to report "hearing voices" while on Newport on business, behavior odd enough that local police turned over a copy of their report to Naval officials to investigate.
Alexis had been arrested twice, once in 2004 and again in 2010, for illegally discharging firearms, though in each case charges were not pursued.
A U.S. defense official said a National Agency Check, a type of background check, was completed on Alexis in August 2007 and he was determined eligible to handle "secret" material in March 2008. Such clearances are valid for 10 years, meaning Alexis had no need to renew his.
Lawmakers on Tuesday said the shooting and the recent case of Edward Snowden, a former National Security Agency contractor who disclosed details about top-secret U.S. spying programs, showed there were serious flaws in the federal government's process of issuing security clearances.
"I want to know who conducted his background investigation, if that investigation was done by contractors, and if it was subject to the same systemic problems we've seen with other background checks in the recent past," Democratic Senator Claire McCaskill said on Tuesday. "Those who lost loved ones and were injured in yesterday's shooting deserve to know the answers to these questions."
Navy Yard officials on Wednesday began allowing workers whose cars had been locked inside the facility since it was evacuated following the shooting to remove their vehicles. The yard otherwise remained open only to base security, firefighters and personnel involved in the investigation.
Alexis fired down from a fourth-floor atrium into a cafeteria where workers were eating breakfast. Some workers recalled barricading themselves in conference rooms or running from the building after they heard shots ring out.
"It was terrifying," said Sam Agger, a 60-year-old program analyst who works supporting Navy radar programs. "It was one of the worst days of my life." (Additional reporting by Margaret Chadbourn; Writing by Scott Malone)