A long-awaited inquiry into a deadly militant attack on the US mission in the Libyan city of Benghazi has slammed State Department security arrangements there as "grossly inadequate".

But the months-long probe also found there had been "no immediate, specific" intelligence about a threat against the mission, which was overrun by dozens of heavily armed militants on September 11 who killed four Americans."Systemic failures and leadership and management deficiencies at senior levels within two bureaus of the State Department resulted in a Special Mission security posture that was inadequate for Benghazi and grossly inadequate to deal with the attack that took place," the report said.The Accountability Review Board (ARB) also concluded "there was no protest prior to the attacks, which were unanticipated in their scale and intensity".The attack has become fiercely politicised, with Republicans skewering the US administration for security failings as well as a possible cover-up over Al Qaeda's role.In the unclassified section of the report, the five-strong board added they believed every effort had been made to rescue ambassador Chris Stevens, who died in the attack - the first US envoy killed on duty since 1979.