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 of a threat against the mission, which was overrun on September 11 by dozens of heavily armed militants who killed four Americans, including the US ambassador to Libya.
"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," said the damning report, issued late on Tuesday.
The Accountability Review Board, appointed by US State Secretary Hillary Rodham Clinton, also concluded "there was no protest prior to the attacks, which were unanticipated in their scale and intensity".
The attacks, in which the consulate and a nearby safe house were targeted, have become fiercely politicised, with Republicans skewering the administration for security failings as well as a possible cover-up over al-Qaeda's role.
The US envoy to the United Nations, Susan Rice, came under relentless Republican fire for saying days after the assault that, according to the best intelligence, it was triggered by a "spontaneous" protest outside the mission. Rice has since been forced to pull out of the running to replace Clinton, who steps down early next year.
In the unclassified section of their 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 in three decades.
Clinton said she accepted "every one" of the 29 recommendations made by the board, which has spent the past three months investigating the events.
She also said the State Department was working with the Pentagon to "dispatch hundreds of additional Marine Security Guards to bolster our posts" and was aiming to train up more diplomatic security personnel.
The report provided "a clear-eyed look at the serious, systemic challenges that we have already begun to fix", Clinton told lawmakers, adding that while everyone at the State Department had a duty to ensure diplomats' safety, "most of all it is my responsibility as secretary of state".
The report noted that the State Department budget accounted for only a very small part of national spending and warned that "Congress must do its part to meet this challenge and provide necessary resources to the State Department to address security risks and meet mission imperatives".
The inquiry "found a pervasive realisation among personnel who served in Benghazi that the Special Mission was not a high priority for Washington", the report said.
Repeated requests for additional support from embassy staff in both Benghazi and the Libyan capital, Tripoli, had been ignored.
The Benghazi mission was also hampered by poor resources, and its reliance on armed "but poorly skilled" local militia.