Navy Yard Shooting
Navy Yard Shooting

U.S. Defense Secretary Chuck Hagel has ordered a thorough review of security at U.S. military bases around the world following Monday's shooting at the Washington Navy Yard that left 13 dead.

Hagel says there remain big questions on how alleged gunman Aaron Alexis - a Navy contractor - managed to retain a security clearance despite a history of mental problems, and how he got a building pass to enter the Washington Navy Yard, where he shot and killed 12 people Monday.

Hagel told reporters at the Pentagon the review he has ordered will try to answer those questions.

“Where there are gaps, we will close them," he said."Where there are inadequacies, we will address them and where there are failures, we will correct them.”

These images released by the FBI show photos of Aa
These images released by the FBI show photos of Aaron Alexis, who police believe was a gunman at the Washington Navy Yard shooting in Washington, Sept. 16, 2013, and who was killed after he fired on a police officer.

Alexis, who was killed during the rampage, was a former Navy reservist and had been honorably discharged from active duty in 2011. He held a security clearance despite a number of police incidents that included firing a gun inside an apartment building and shooting out someone's car tires.

Alexis' mental state has emerged as a central factor in the investigation.  Police say he recently reported hearing voices at hotel where he was staying.  He had been receiving mental health treatment under a veterans' program.

However, neither the police incidents - which did not result in convictions - nor the psychiatric episodes prompted any action to suspend his clearance.  

Hagel said the investigation will seek to find out why the U.S. military did not act on the tips.  

“Why they didn't get picked up, why they didn't get incorporated into the clearance process, what he was doing, those are all legitimate questions that we're going to be dealing with," said Hagel.

Members of Congress are calling for a review of hiring practices at the Department of Defense, and specifically of the background checks that were done on Alexis.

The Navy Yard shooting was the worst on a military facility in the United States since 2009 when army psychiatrist Nidal Hasan massacred 12 soldiers and one civilian at Fort Hood, Texas.