Washington, Reuters—Washington authorities questioned on Tuesday how a US military veteran with a history of violence and mental problems could have gotten clearance to enter a Navy base where he killed 12 people before police shot him dead.

The suspect, Aaron Alexis, 34, a Navy contractor from Fort Worth, Texas, entered Washington Navy Yard on Monday morning and opened fire, spreading panic at the base just a mile and a half from the US Capitol and three miles from the White House.

Investigators are still trying to determine the shooter’s motive. Alexis had been given clearance to enter the base on the Anacostia River, despite two gun-related brushes with the law and a discharge from the Navy Reserve in 2011 after a series of misconduct issues.

A federal law enforcement source told Reuters Alexis had a history of mental problems but gave no details. CNN reported that Alexis had contacted two Veterans Administration hospitals recently and was believed to be seeking psychological help.

“It really is hard to believe that someone with a record as checkered as this man could conceivably get, you know, clearance to get…credentials to be able to get on the base,” Washington Mayor Vincent Gray told CNN.

He said automatic US budget cuts known as sequestration could have led to skimping on vetting that would have barred Alexis from the heavily guarded base.

“Obviously, 12 people have paid the ultimate price for whatever was done to have this man on base,” Gray said.

The incident prompted immediate calls for reviews of base security procedures. Congressman Michael Turner called for Defense Department officials to release information on an inspector general’s audit of its system for controlling civilian workers’ access to military bases.

The Navy may have “implemented an unproven system in order to cut costs,” Turner, an Ohio Republican, said in a letter dated Monday to Lynne Halbrooks, the Pentagon’s acting inspector general.

“Potentially numerous felons may have been able to gain unrestricted access to several military installations across the country,” said Turner, a member of the House Armed Services Committee.

Military personnel are generally banned from carrying weapons on military installations in the United States but most people with proper credentials are not routinely checked for firearms.

Eight people were hurt, including three who were shot. Police have identified seven of those killed, with identification of the others pending notification of relatives.

The three gunshot victims, including a Washington police officer severely wounded in the legs, were doing well, Janis Orlowski, the chief medical officer at Washington Hospital Center, told NBC’s “Today” program.

Police shot Alexis in a gun battle after he entered the Naval Sea Systems Command headquarters about 8:20 a.m. (1220 GMT) and started picking off victims in a cafeteria from a fourth-floor atrium, witnesses said.

Alexis was armed with an AR-15 military-style assault rifle, a double-barreled shotgun and a handgun, a federal law enforcement source said.

The shooting was the worst attack at a US military installation since Army Major Nidal Hasan opened fire on unarmed soldiers at Fort Hood, Texas, in 2009, killing 13 people and wounding 31.

Alexis, a contract employee, had legitimate access to the Navy Yard and used a valid pass, the FBI said. Authorities have not addressed how he could have gotten weapons onto the base.

Alexis, a one-time Texas resident who was known to worship at a Buddhist temple, served full time in the US Navy Reserve from May 2007 to January 2011, becoming an aviation electrician.

He was recently hired as a civilian information technology contractor to work on the Navy and Marine Corps intranet. He was given a security clearance classified as “secret,” his company’s chief executive told Reuters.

Alexis was arrested on September 4, 2010, in Fort Worth, Texas, on a misdemeanor charge of discharging a firearm. The case was dropped when investigators determined he was cleaning his gun and it accidentally fired, Tarrant County prosecutors said.

He was also arrested in Seattle in 2004 for shooting out a construction worker’s car tires in an anger-fueled “blackout” triggered by perceived “disrespect,” according to the Seattle Police Department.

The base was closed to all but essential personnel on Tuesday as police continued their investigation. Military police were stationed at the four entrances, checking the identifications of the employees who were being allowed back in. Other personnel milled around outside, hoping to retrieve cars that remained locked inside the gates.

“I still don’t know when I can get my car,” said a woman who identified herself only as Linda. She declined to give her last name because she was not supposed to return to work on Tuesday.

She said she had had no Internet access on Monday due to security in her building and only found out about the shooting from her family. “My phone just blew up,” Linda said.