WASHINGTON: A long-awaited inquiry into a deadly militant attack on the US mission in the Libyan city of Benghazi late on Tuesday 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 damning 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 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 since 1979.

US Secretary of State Hillary Clinton said she accepted “every one” of the 29 recommendations made by the ARB, which has spent the last three months investigating the events of that night.

She also said the State Department was working with the Pentagon to “dispatch hundreds of additional Marine Security Guards to bolster our posts.” Repeated requests for additional support from embassy staff in both Benghazi and the Libyan capital Tripoli had been ignored, the report said.

The inquiry “found a pervasive realization among personnel who served in Benghazi that the Special Mission was not a high priority for Washington,” the report added.

The Benghazi mission was also hampered by poor resources and the 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 Tuesday to implement the report's recommendations.

The classified findings of the investigation were Tuesday sent to members of two House and Senate committees.

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

The Benghazi report was sent by courier to Clinton at home on Monday, and she has read the highly-anticipated findings. But she will not be testifying herself this week after falling ill and being told by doctors to rest.