The Benghazi consulate had been hampered by a lack of resources, the report said
Three state department officials have resigned after a report into the attack on the US consulate in Benghazi, Libya, US media say.
They include Eric Boswell, head of the Bureau of Diplomatic Security, and one of his deputies, Charlene Lamb.
Raymond Maxwell, deputy assistant secretary for the Maghreb, was said to be the third resignation.
The report said "grossly inadequate" Benghazi security led to the deaths of the US ambassador and three others.
It singled out both state department bureaus for their apparent lack of co-operation and ensuing confusion with regards to protection.
However, the report did not suggest disciplinary action be taken against any individuals.
Ambassador Christopher Stevens and three other US officials died in the attack on 11 September.
Mr Stevens died of smoke inhalation when he was trapped alone in the burning building after armed men had stormed the compound.
Despite "a lack of transparency, responsiveness, and leadership" among certain senior state department officials, the review found no "reasonable cause" that any specific individuals had "engaged in misconduct or wilfully ignored" their responsibilities.
It also said there had been "no immediate, specific" intelligence about the 11 September attack or threats to the consulate.
The probe concluded that the US personnel had "performed with courage and readiness to risk their lives to protect their colleagues in a near-impossible situation".
But the Benghazi mission had nevertheless been hampered by a lack of resources.
Its reliance on armed "but poorly skilled" local militiamen and contract guards was "misplaced," the report said.
In a letter to Congress, US Secretary of State Hillary Clinton said she accepted all 29 of the recommendations put forward in the report.
She outlined some steps the agency would take, including sending hundreds of US Marines guards to missions abroad and assigning a state department official to oversee "high-threat posts".
In addition, she said the state department would request more funding from Congress to make improvements to security.
The attack on the consulate triggered a major political row, and the Accountability Review Board was charged with investigating.
Days after the attack, US ambassador to the UN Susan Rice said the attack seemed to have developed out of protests over an anti-Islamic film.
But later intelligence reports suggested it was possibly tied to al-Qaeda affiliates.