Washington: A long-awaited inquiry into a deadly militant attack on the US mission in the Libyan city of Benghazi late Tuesday 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.

“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 attacks, in which the consulate and a nearby annexe were targeted, have become fiercely politicised, with Republicans skewering the administration for security failings as well as a possible cover-up over Al Qaida’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 US Secretary of State Hillary Clinton, who steps down early in 2013.

In the unclassified section of their report, the five-strong board said 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 ARB, which has spent the last 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 more diplomatic security personnel.

The report provided “a clear-eyed look at the serious, systemic challenges that we have already begun to fix,” Clinton said in a letter to lawmakers, adding that while everyone at State had a duty to ensure diplomats’ safety, “most of all it is my responsibility as secretary of state”.

Clinton also backed the report’s findings urging Congress to support moves to realign the department’s 2013 budget request to help reinforce its diplomatic outposts, in her letter provided to journalists.

The New York Times reported the State Department is asking Congress to transfer $1.3 billion (Dh4.77 billion) from contingency funds that had been allocated to Iraq.

This would include $553 million for the extra Marines, $130 million for diplomatic security personnel, and $691 million for improving security at diplomatic missions, the daily said.

The ARB report warned “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”.

Repeated requests for additional support from embassy staff in both Benghazi and the Libyan capital Tripoli had been ignored, the report said, adding that “the special mission was not a high priority for Washington”.

But it said Stevens made the decision to travel to Benghazi “independently of Washington, per standard practice” and because of his in-depth knowledge of Libya, Washington gave “unusual deference to his judgements”.

The Benghazi mission was also hampered by poor resources, and its reliance on armed “but poorly skilled” local militiamen from the February 17 Martyrs Brigade as well as local unarmed staff hired by a British company, Blue Mountain, was “misplaced,” it said.

Clinton has now entrusted Deputy Secretary Tom Nides with heading up a team which met for the first time on Tuesday to implement the report’s recommendations.

The unclassified section of the report was posted on the State Department website late Tuesday, while the classified findings were sent to members of two House and Senate committees.

ARB chairman, veteran diplomat Thomas Pickering, and vice-chairman Admiral Mike Mullen, the former chairman of the Joint Chiefs of Staff, will brief those lawmakers on Wednesday behind closed doors.

The Benghazi report was sent by courier to Clinton at home on Monday, but she will not be testifying herself, as had been planned, after falling ill and being told by doctors to rest.