WASHINGTON  - The State Department's decision to keep the US mission in Benghazi open despite inadequate security and increasingly dangerous threat assessments before it was attacked in September was a "grievous mistake," a Senate report said on Monday. The Senate Homeland Security Committee's report about the September 11 attacks on the US mission and a nearby annex, which killed four Americans, including the US ambassador to Libya, faulted intelligence agencies for not having enough focus on Libyan extremists. It also faulted the State Department for waiting for specific warnings instead of acting on security.

The assessment follows a scathing report by an independent State Department accountability review board that resulted in a top security official and three others at the department stepping down. The attack, in which US Ambassador Christopher Stevens died, has put diplomatic security practices at posts in insecure areas under scrutiny and raised questions about whether intelligence on terrorism in the region was adequate.