The report also criticised the Libyan government's response to the attack, characterising it as "profoundly lacking"
Security at the US consulate in the Libyan city of Benghazi was "grossly inadequate" when an attack killed the US ambassador, an official report says.
There were systemic failures at the state department, but no individual official ignored their duties, it adds.
US Secretary of State Hillary Clinton has accepted all 29 recommendations contained in the report.
The 11 September attack saw Ambassador Christopher Stevens, as well as three other US officials, killed.
The ambassador died of smoke inhalation when he was trapped alone in the burning consulate building, after armed men had stormed the compound.
The assault triggered a major political row over who knew what and when. As a result, an independent panel - the Accountability Review Board- was charged with investigating the incident.
The report identified the state department's Bureau of Diplomatic Security and Bureau of Near East Affairs for criticism over their apparent lack of cooperation and ensuing confusion with regards to protection.
"Systematic 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 panel concluded.
However, the review has not suggested disciplinary action be taken against any individuals. 'Profoundly lacking'
Despite "a lack of transparency, responsiveness, and leadership" among certain senior state department officials, the report 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.
In a letter to congressional committees, Mrs Clinton said she had ordered the state department to implement the investigation's findings "quickly and completely".
She outlined some steps the agency would take, including sending hundreds of Marine guards to US missions abroad and assigning a state department official to oversee "high threat posts".
In addition, Mrs Clinton said the state department would request more funding from Congress to make improvements to security.
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. The reliance on armed "but poorly skilled" local militiamen and contract guards was "misplaced," the report said.
"The solution requires a more serious and sustained commitment from Congress to support State Department needs which constitute a small percentage of the full national budget and that spent for national security," according to the review.
The probe also criticised the Libyan government's response to the attack, characterising it as "profoundly lacking".
The Obama administration's handling of the attack in Benghazi has become the focus of Republican criticism.
Officials initially said the attack had developed out of protests against an anti-Muslim video.
But later intelligence reports suggested it was possibly tied to al-Qaeda affiliates.