WASHINGTON (AP) - The leaders of an independent panel that blamed systematic State Department management and leadership failures for gross security lapses in the deadly Sept. 11 attack on a U.S. diplomatic mission in Libya will explain their findings to Congress on Wednesday.

The two most senior members of the Accountability Review Board are set to testify behind closed doors before the House and Senate foreign affairs committees on the classified findings of their harshly critical report.

An unclassified version released late Tuesday said serious bureaucratic mismanagement was responsible for the inadequate security at the mission in Benghazi where the U.S. ambassador and three other Americans were killed.

"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.

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

Wednesday's classified testimony from the review board - retired Ambassador Thomas Pickering and a former Joint Chiefs of Staff chairman, Adm. Mike Mullen - will set the stage for open hearings the next day with Deputy Secretary of State William Burns, who is in charge of policy, and Deputy Secretary of State Thomas Nides, who is in charge of management.

Secretary of State Hillary Rodham Clinton was to have appeared at Thursday's hearing but canceled after fainting and sustaining a concussion last week while recovering from a stomach virus that dehydrated her. Clinton is under doctors' orders to rest.

In a letter that accompanied the transmission of the report to Capitol Hill, Clinton thanked the board for its "clear-eyed, serious look at serious systemic challenges" and said she accepted all of its 29 recommendations to improve security at high-threat embassies and consulates.

She said the department had already begun to implement some of the recommendations. They include increasing by several hundred the number of Marine guards stationed at diplomatic missions throughout the world; relying less on local security forces for protection at embassies, consulates and other offices; and increasing hiring and deployment of highly trained Diplomatic Security agents at at-risk posts.

Clinton agreed with the panel's finding that Congress must fully fund the State Department's security initiatives. The panel found that budget constraints in the past had led some management officials to emphasize savings over security, including rejecting numerous requests from the Benghazi mission and the embassy in Tripoli for enhanced protection.

It 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.

But it 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.

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

It did confirm, though, that contrary to initial accounts, there was no protest outside the consulate. It 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 that had begun in Cairo earlier that day over an American-made, anti-Islamic film. Those comments came after evidence already pointed to a distinct militant attack.

United Nations 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 brouhaha in the heat of the presidential campaign eventually led her to withdraw her name from consideration to replace Hillary Rodham Clinton as secretary of state in President Barack Obama's second term.

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" and absolved the military from any blame. "There was simply not enough time for armed U.S. military assets to have made a difference," it said.

The report also discounted speculation that officials in Washington had refused appeals for additional help after the attack had begun.

"The Board found no evidence of any undue delays in decision making or denial of support from Washington or from the military combatant commanders," it said. To the contrary, the report said the evacuation of the dead and wounded 12 hours after the initial attack was due to "exceptional U.S. government coordination and military response" that helped save the lives of two seriously wounded Americans.