WASHINGTON (AP) ― An independent panel charged with investigating the deadly Sept. 11 attack in Libya that killed the U.S. ambassador and three other Americans has concluded that systematic management and leadership failures at the State Department led to “grossly” inadequate security at the mission in Benghazi.

“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 said.

The report singled out the Bureau of Diplomatic Security and the Bureau of Near East Affairs for criticism, saying there appeared to be a lack of cooperation and confusion over protection at the mission in Benghazi, a city in eastern Libya that was relatively lawless after the revolution that toppled Libyan strongman Moammar Gadhafi.

Despite those failures, the Accountability Review Board determined that no individual officials ignored or violated their duties and recommended no disciplinary action now. But it also said poor performance by senior managers should be grounds for disciplinary recommendations in the future.

The report appeared to break little new ground about the timeline of the Benghazi attack during which Libyan Ambassador Chris Stevens, information specialist Sean Smith and former Navy SEALs Glen Doherty and Tyrone Woods -- who were contractors working for the CIA -- were killed. Stevens’ slaying was the first of a U.S. ambassador since 1988.

But it confirmed that contrary to initial accounts, there was no protest outside the consulate and said responsibility for the incident rested entirely with the terrorists who attacked the mission.

In the immediate aftermath of the attack, administration officials linked the attack to the spreading protests over an anti-Islamic film made in the U.S. that had begun in Cairo, Egypt, earlier that day. Those comments came after evidence already pointed to a distinct militant attack. U.N. Ambassador Susan Rice appeared on numerous TV talk shows the Sunday after the attack and used the administration talking points linking it to the film. An ensuing dispute in the heat of the presidential campaign eventually led Rice to withdraw her name from consideration to replace Hillary Rodham Clinton as secretary of state in President Barack Obama’s second term.

The review board determined that there had been no immediate, specific tactical warning of a potential attack on the 11th anniversary of the Sept. 11, 2001, terrorist attacks. However, the report said there had been several worrisome incidents in the run-up to the attack that should have set off warning bells.

While criticizing State Department management in Washington along with the local militia force and contract guards that the mission depended on for protection, the report said U.S. personnel on the ground in Benghazi “performed with courage and readiness to risk their lives to protect their colleagues in a near-impossible situation.”

It said the response by Diplomatic Security agents on the scene and CIA operatives at a nearby compound that later came under attack itself had been “timely and appropriate” but absolved the military from any blame. “There was simply not enough time for armed U.S. military assets to have made a difference.”

The State Department sent a classified version of the report to lawmakers on Tuesday and shortly thereafter released an unclassified version. The report made 29 recommendations to improve embassy security, particularly at high threat posts. In an accompanying letter to Congress, Clinton thanked the board for its “clear-eyed, serious look at serious systemic challenges” and said she accepted all the recommendations.

Some of those challenges were revealed in earlier congressional hearings when several State Department officials discussed competing demands for security and cost prohibitions.