An independent panel charged with investigating the deadly Sept. 11 attack in Libya that killed a U.S. ambassador and three other Americans has concluded that systematic management failures at the State Department led to inadequate security that left the diplomatic mission vulnerable.

Despite those failures, the Accountability Review Board determined that no individual American officials ignored or violated their duties and found no cause for any disciplinary action. The board found that contrary to initial reports, there was no protest outside the mission and blamed the incident entirely on terrorists.

The State Department sent a classified version of the report to lawmakers Tuesday and released an unclassified version later Tuesday. The report makes 29 recommendations to improve embassy security. Secretary of State Hillary Rodham Clinton said she accepted them all.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

An independent review board on Tuesday presented to Congress its report on the Sept. 11 attack in Libya that killed a U.S. ambassador and three other Americans, findings the Obama administration hopes will bolster its assertion that diplomats took all reasonable measures to anticipate and respond to the violence, and end months of finger-pointing and recriminations over whether the deaths could have been avoided.

Diplomats and intelligence officers alike have spoken about the rising risk in Benghazi and growing debate over how to improve security before the attack, set against Ambassadors Chris Stevens' decision to keep the Benghazi diplomatic post open and even visit there on Sept. 11.

Late that evening, militants overran the lightly defended U.S. Consulate, setting fire to it and ultimately killing Stevens and information specialist Sean Smith. Militants later fired mortars at the CIA safe house where survivors had taken refuge, killing Americans Glen Doherty and Tyrone Woods, both former Navy SEALs working for the CIA who had come to help rescue the diplomats.

Retired Ambassador Thomas Pickering and former Chairman of the Joint Chiefs Adm. Mike Mullen led the independent review, studying cables, video and intelligence and interviewing some survivors. Their review will also include their recommendations on how to keep it from happening again.

The classified section of their report was sent to Capitol Hill on Tuesday afternoon, a State Department official said. As it was being transmitted, the department began to implement its recommendations, with a team of senior diplomats convening to look at how to execute them.

An unclassified summary of the report is expected to be made public on Wednesday. On Thursday, the State Department's two deputy secretaries will testify in open sessions before House Foreign Affairs Committee and Senate Foreign Relations Committee.

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

The secretary of state is required by law to convene an Accountability Review Board when U.S. officials are killed overseas in the line of duty. The secretary is not, however, required to make its findings public or send classified details to lawmakers. Still, Clinton has pledged to share as much detail as possible.

ARB inquiries are designed to look at security procedures before and during the incidents in question, and make recommendations on how they can be prevented in the future. Previous boards, notably the ones established after the 1998 bombings of the U.S. embassies in Kenya and Tanzania, have found broad fault across all branches of government, including Congress, but have not targeted individuals for blame.

The Benghazi report is widely expected to draw similar conclusions, finding that security was clearly lacking at the mission but that blame for that rests within the government bureaucracy and that there is no way to prove that additional protection could have stopped the attack.

The Benghazi incident became politically heated in the last weeks of the tight presidential campaign, with U.N. Ambassador Susan Rice criticized for saying in television interviews that the attack was part of a protest over an anti-Islamic film that had started in Cairo earlier that day. Her comments came days after the administration had intelligence pointing to a militant attack. The political fallout is not within the review board's mandate and its report is not expected to mention the controversy, except possibly in passing.

But the incident also sparked debate on the larger question of how U.S. diplomats and intelligence officers can do their jobs in unstable environments, as al-Qaida spreads across Africa, without also expanding their security. Diplomats have said that overreacting to the attack could produce what some are calling a "Benghazi effect" that would wall them off from the people they are supposed to be engaged with.

Though called a consulate, the U.S. diplomatic facility in Benghazi was so small it was only lightly staffed by a skeleton crew of diplomats and guarded by a local Libyan force. Its small size was partly because there was not yet much diplomacy to do in a city still rebuilding after the revolution that toppled Libyan strongman Moammar Gadhafi, but also because the consulate's role was in part to provide cover for the larger intelligence gathering mission in the region.

Most of the roughly two dozen Americans based in Benghazi worked for the CIA, some helping the State Department track and buy up the tens of thousands of shoulder-fired anti-aircraft missile systems known as MANPADS _ man-portable-air-defense-systems _ that have made their way from Gadhafi's arsenal to the open market, trying to keep them out of the hands of al-Qaida sympathizers throughout the region. That work included developing a network of tipsters in the port city who would watch for weapons shipments headed out of the country.

The officers were also trying to track al-Qaida's recruitment efforts in Benghazi and the small city of Darna, a couple hundred miles up the coast. Since the fall of the Gadhafi regime, Darna has become a hotbed of militant groups, such as al-Qaida in the Islamic Maghreb and the militant group Ansar al-Shariah, the lead suspect in the Benghazi attack, according to two U.S. officials briefed on the intelligence. They spoke on condition of anonymity because they were not authorized to speak publicly.

The drone that flew over Benghazi the night of the attack to give policymakers back in Washington a view of the violence had been patrolling over Darna, officials said.

Some Libyan militiamen did respond during the attack, the officials said, but not in numbers sufficient or with enough organization to repel the mob. It was the CIA rescue team that came in, guns blazing, to rescue the diplomats, while a second pick-up team of two special operations troops and CIA security personnel flew in on a hastily chartered plane from Tripoli, convinced Libyan militiamen at the airport to give them transport and rushed in to help protect the diplomats, who by then had reached a CIA safe house in the town, where they were attacked again.

The combined CIA teams held off a brief gunfire attack at the second location, so the militiamen fired mortars at the building, killing the two ex-Navy SEALs who were part of the CIA security team. The combined CIA team returned fire and the mortar fire ceased, and they were able to get the diplomats to the airport.

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