A report into the attack on the US Consulate in Benghazi, Libya, has found that systemic failures at the State Department led to insufficient security at the compound.
US Ambassador Christopher Stevens and three other Americans died in the 11 September attack.
The report also found that the Libyan response to the attacks was "profoundly lacking".
It specifically faulted the State Department's Bureau of Near Eastern Affairs and its Bureau of Diplomatic Security.
The report cited "leadership and management" deficiencies at the two bureaus, poor coordination among officials in Washington, and "real confusion" on the ground over who had the responsibility, and the power, to make decisions that involved policy and security concerns.
The report's harsh assessment seemed likely to tarnish the four-year tenure of Secretary of State Hillary Clinton, who said in a letter accompanying the review that she would adopt all of its recommendations.
The incident has raised questions about the adequacy of security at US embassies around the globe and where to draw the line between protecting US diplomats in dangerous places, while giving them enough freedom to do their jobs.
The political fallout in the US following the attack led to the withdrawal of Susan Rice for consideration for the post of Secretary of State.
She was widely tipped as a front-runner to replace Mrs Clinton when she steps down in the new year, but said she wished to avoid a potentially disruptive Senate confirmation process.
Republican politicians had criticised Ms Rice for comments she made on several television talk shows in the aftermath of the attack, in which she said preliminary information suggested the assault was the result of protests over an anti-Muslim video made in California rather than a premeditated strike.
The review, however, concluded that no protest took place before the attack.