The Pentagon missed chances to prevent the shooting at the Navy Yard last September, an internal review of the incident found.
A report, released Tuesday, examined the lead up September 16, when former Navy reservist Aaron Alexis entered the Defense Department's installation in southeast Washington and shot dead 12 Navy civilians and contractors before being killed by police.
The review found the security clearance review system ill-equipped to have identified Alexis as a potential threat, despite "a pattern of misconduct and disturbing behavior" that ought to have raised alarms.
Instead, the Office of Personnel Management, which is responsible for overseeing the review system of security clearances for federal workers, approved Alexis' clearance even though it "was missing critical information."
His commanders neglected to record "multiple incidents of adverse behavior of his time on active duty," and therefore, his employer, The Experts Inc., "had no insight into Alexis's chronic personal conduct issues during his Navy service" when they hired him.
Secretary of Defense Chuck Hagel Tuesday responded to the report by calling for a "continuous evaluation" system to regularly screen federal employees with security clearance and called for the creation of an "insider threat management and analytical center."
The shooting at the Navy Yard is the second-deadliest mass shooting at a military property, behind the 2009 shooting at Fort Hood in which Army Maj. Nidal Hasan shot dead 13 people and wounded 30 others.