SECURITY at the US mission in the Libyan city of Benghazi was grossly inadequate to deal with an attack that killed a US ambassador and three others because of failures within the State Department, an official inquiry found.

The review cited leadership and management deficiencies at two department offices, poor coordination among officials and confusion in Washington and in the field over who had the power to make decisions that involved policy and security concerns.

The attack killed Ambassador Christopher Stevens and three other Americans. The report could tarnish the tenure of Secretary of State Hillary Clinton, who said she would adopt its recommendations.

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The board criticised the Bureau of Near Eastern Affairs, the regional office responsible for the Middle East and North Africa, and the Bureau of Diplomatic Security, its law enforcement and security arm.

The board said US intelligence provided no specific tactical warning of the attack and that there was "little understanding of militias in Benghazi and the threat they posed to US interests" in the city, where the central government has little influence.