The US Department of Defense has ordered a security review at its facilities worldwide, including security clearances. The move follows the massacre at Washington Navy Yard, where Aaron Alexis killed 12 people.

"Time frame, the depth of the clearance, kind of clearance, access, different clearances given to individuals. We're going to look at all that," Defense Secretary Chuck Hagel said during a news conference..

"Obviously the longer clearances go without review, there is some jeopardy to that. There's no question about it. So we're going to take a look at every one of those components."

Hagel has ordered the military to look at all existing security measures, check their sufficiency and to determine what other measures may be needed.

The Defense Secretary also tasked an independent panel to undertake the same reviews.

He acknowledged that there were some “red flags” about suspect Aaron Alexis.

It still remains unclear how the 34-year old shooter maintained a security clearance despite numerous violent incidents, run-ins with the law and serious mental illness. Earlier, US lawmakers demanded a review into that issue.

"I want to know who conducted his [Alexis'] 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," US Senators Claire McCaskill said in a statement to Reuters on Tuesday.

The shooting incident has prompted the US Secretary of Defense to seek a report on American military installations around the world. A senior Pentagon official said that Hagel will work with senior officials to create a process for the review, which was formally announced Wednesday.