Senate Foreign Relations Chairman John Kerry, D-Mass., speaks to reporters following a closed-door briefing on the investigation of the deadly Sept. 11 attack on the U.S. consulate in Benghazi, Libya, at the Capitol in Washington, Wednesday, Dec. 19, 2012. An Accountability Review Board's report indicates 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. (AP Photo/J. Scott Applewhite)

WASHINGTON - U.S. lawmakers will demand answers again Thursday on why security was "grossly inadequate" when militants stormed the country's diplomatic mission in Benghazi, Libya, on Sept. 11, killing the ambassador and three other Americans, and on why the military failed to respond faster during the nine-hour assault.

Members of the Senate and House foreign affairs committees will question 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, at back-to-back congressional hearings.

Lawmakers also insist that Secretary of State Hillary Rodham Clinton testify in the coming weeks despite her plan to leave the administration. Clinton had been scheduled to testify Thursday, but she cancelled after fainting and sustaining a concussion last week while recovering from a stomach virus. She is under doctors' orders to rest.

Many Republicans have used the Libya attack to criticize the Obama administration and its response. Their opposition to U.N. Ambassador Susan Rice as a possible candidate to succeed Clinton, after Rice blamed the attack not on terrorism but on protests against an anti-Muslim film, led to Rice taking herself out of the running, even after Rice explained that she had relied on talking points drawn up by intelligence agencies.

The public testimony Thursday comes two days after an independent review panel issued a blistering report blaming management failures at the State Department for the lack of security. Fallout from the report forced four State Department officials to step down on Wednesday.

"Why, if we quickly did find out it was in part a terrorist attack, why wasn't there better security on that evening with the ambassador in Benghazi and in the consulate and what do we need to do to make sure?" asked Republican Sen. Johnny Isakson, a member of the Foreign Relations Committee.

"People keep forgetting that was about a nine-hour attack moving from the consulate to the annex. We had already called up troops from Fort Bragg (North Carolina) and got them to Sicily before the attack was over," he said. "We knew it was a big-time attack. We flew in two planes from Djibouti, additional assets from Croatia. We need to find out who knew what when."

U.S. Ambassador Chris Stevens was killed, along with information specialist Sean Smith and former Navy SEALs Glen Doherty and Tyrone Woods, who were contractors working for the CIA. Stevens was the first U.S. ambassador killed in the line of duty since 1979.

An unclassified version of the report by the Accountability Review Board concluded that "systematic" management and leadership failures at the State Department led to "grossly inadequate" security at the mission in Benghazi.

The report singled out the Bureau of Diplomatic Security and the Bureau of Near East Affairs, saying there appeared to be a lack of co-operation 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.

Obama administration officials said those who resigned were Eric Boswell, assistant secretary of state for diplomatic security; Charlene Lamb, deputy assistant secretary responsible for embassy security, and Raymond Maxwell, deputy assistant secretary of state who oversees the Maghreb nations of Libya, Algeria, Tunisia and Morocco. The officials spoke on condition of anonymity because they weren't authorized to discuss personnel matters publicly.

Some of the three may have the option of being reassigned to other duties, the officials said.

State Department spokeswoman Victoria Nuland said the department had accepted the resignations of four people: Boswell and three others she declined to identify.

The resignations did little to mollify lawmakers who will question Burns and Nides and want to question Clinton.

"She is ultimately responsible for the department and U.S. posts around the world, her testimony before the Senate Foreign Relations Committee is indispensable to any effort to address this failure and put in place a process to ensure this never happens again," said Republican Sen. Marco Rubio.

The report's findings underscore a fundamental problem the State Department has been trying to address for decades without success: How to protect diplomats while allowing them to perform their duties to reach out to foreign governments and the public to promote U.S. interests and values.

In a letter to Congress, Clinton said "our diplomats cannot work in bunkers and do their jobs."

"When America is absent, especially from dangerous places, there are consequences," she said. "Extremism takes root, our interests suffer, and our security at home is threatened. We must accept a level of risk to protect this country we love and to advance our interests and values around the world."

The American Foreign Service Association, the union that represents U.S. diplomats, said it agreed. It welcomed the findings and accepted the board's 29 recommendations for improving embassy security, particularly at high-threat posts.

"There is inherent risk in the practice of active and effective diplomacy and our diplomatic personnel will always be exposed to a degree of harm in the line of duty," the association said in a statement. "It is our responsibility to do all we can to minimize the risk and balance it with the importance of the mission and to ensure that the missions we undertake have the personnel and financial resources to achieve policy goals."

At the State Department, retired Adm. Mike Mullen, co-chairman of the review board, said the mission's security fell through bureaucratic cracks caused in part because buildings were categorized as temporary. Budget constraints also led some officials to be more concerned with saving scarce money than in security, the report said.

The other co-chairman, retired ambassador Thomas Pickering, said personnel on the ground in Benghazi had reacted to the attack with bravery and professionalism. But he said the security precautions were "grossly inadequate" and the contingent was overwhelmed by the heavily armed militants.

"They did the best they possibly could with what they had but what they had wasn't enough," Pickering said.

Pickering and Mullen spoke shortly after briefing members of Congress in private.

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Associated Press writers Donna Cassata and Matthew Lee contributed to this report.

News from © The Associated Press, 2012