U.S. House and Senate committees are questioning top State Department officials about the deadly September 11 attack in Libya after an independent panel concluded that security at the U.S. mission in Benghazi was "grossly inadequate" at the time of the incident.
Deputy Secretaries of State William Burns and Thomas Nides are testifying before the House and Senate foreign affairs committees, on Thursday.
Their appearances come a day after the findings in the report from the Accountability Review Board prompted a personnel shake-up at the State Department.
On Wednesday, a State Department spokeswoman said Secretary of State Hillary Clinton has accepted "Eric Boswell's decision to resign as Assistant Secretary for Diplomatic Security." Spokeswoman Victoria Nuland also said three other people had been "relieved of their current duties." Nuland did not name the other individuals. She said all four had been placed on administrative leave.
The Benghazi attack killed the U.S. ambassador and three other Americans.
Clinton had been scheduled to testify before the committees about the attack on Thursday. However, she is under doctor's order to rest after becoming ill, last week.
In a Thursday statement, Senate Foreign Relations committee member Marco Rubio said he wished Clinton a "speedy recovery." But the Republican lawmaker added that Clinton was "ultimately responsible" for the State Department and U.S. posts around the world.
He said her testimony before the committee was "indispensable to any effort to address this failure."
The accountability board said senior-level "systematic failures and management deficiencies" within two State Department bureaus led to the failed protection of the consulate.
The board also said the number of diplomatic security staff members in Benghazi at the time of the attack was "inadequate," in spite of repeated requests from diplomats in Libya for additional staffing.
Retired Ambassador Thomas Pickering and a former Joint Chiefs of Staff chairman, retired Admiral Mike Mullen, headed the investigation.
At the State Department Wednesday, Pickering said those defending the consulate during the attack "did their best that they possibly could with what they had, but what they had was not enough."
Mullen added that "no one knowingly ignored" his or her responsibilities, but certain senior officials lacked "leadership."
Deputy Secretary of State Bill Burns said the problems found were "unacceptable" and asked Congress for funds to reinforce vulnerable facilities.
"We have learned some very hard and painful lessons in Benghazi. We are already acting on them. We have to do better. We have to do more to constantly reduce the risks that people face and make sure they have the resources they need," he said. "We owe them -- to our colleagues that lost their lives in Benghazi - we owe it to the security professionals who acted with such extraordinary heroism that awful night to protect them."
The panel's recommendations include increased security at temporary facilities in high-risk areas.
The group also urged the State Department to lengthen the duty assignments for program and security personnel at high-risk posts. It said the "short-term, transitory nature" of staffing at the Benghazi mission had resulted in "diminished institutional knowledge" and lack of continuity.
The four Americans killed in the Benghazi attack included Ambassador Christopher Stephens. He was the first U.S. ambassador killed in the line of duty since 1988.