The attack left four Americans, including U.S. Ambassador J. Christopher Stevens,dead.
“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 read.
It said “there was no protest prior to the attacks,” which the report described as “unanticipated in their scale and intensity.”
The report also cited as “inadequate” the Bureau of Diplomatic Security security staff in Benghazi on the day of the attack and in the months and weeks leading up to it, “despite repeated requests from Special Mission Benghazi and Embassy Tripoli for additional staffing.”
Before the report was released, a source who had read it told CNN that senior management in charge of diplomatic security “does not come out well at all.”
Assistant Secretary of State Eric Boswell is the head of diplomatic security, and Deputy Assistant Secretary of State Charlene Lamb oversaw State Department decisions on security at the diplomatic outpost. Lamb testified before Congress about the security precautions; documents show Lamb denied repeated requests for additional security in Libya.
The Accountability Review Board completed its investigation into the matter this week and sent a copy to Secretary of State Hillary Clinton for review. A classified version of the report was delivered Tuesday afternoon to members and staff of the committees on Capitol Hill that have jurisdiction over the State Department.
The unclassified version was released Tuesday night.
Veteran diplomat Thomas Pickering and former Joint Chiefs of Staff Chairman Adm. Mike Mullen, both members of the review board, will brief members of the House Foreign Affairs and Senate Foreign Relations Committees in a classified setting about the report on Wednesday.
State Department spokeswoman Victoria Nuland told reporters Tuesday that Clinton, who is home recovering from a stomach virus and concussion, wrote a letter to members of Congress that will accompany the report being sent to Capitol Hill.
Clinton ordered the review in the aftermath of the attack. Such reports are mandated by Congress when Americans working on behalf of the United States government are killed overseas.
Read more: Benghazi attack back in the spotlight
In a notice sent to all State Department employees Tuesday, the department explained how the report would be implemented once it was released.
“To implement the Board’s recommendations, the Secretary has directed the Deputy Secretary for Management and Resources to lead the implementation team, supported by the Under Secretary for Political Affairs, the Under Secretary for Management, the Director General of the Foreign Service, the Executive Secretary and the Deputy Legal Advisor,” the notice said.
Employees were told that the implementation team had met Tuesday and would continue to do so regularly to implement the recommendations of the board.
The politics surrounding the events that led to the report have claimed one political casualty, with U.S. Ambassador to the United Nations Susan Rice last week pulling her name from consideration to succeed Clinton. Some Republican senators had said they would put a hold on her nomination if President Barack Obama had submitted it, based on comments Rice made in the days after the attack.
In place of Clinton, Deputy Secretaries of State William Burns and Thomas Nides will testify before the House and Senate committees Thursday.