The scandal over the Benghazi attack grew as a top State Department official resigned and three others were suspended when a probe uncovered major security failures and mismanagement.
The news came amid a clamor of calls for Secretary of State Hillary Clinton to testify to US lawmakers about the attack on the US consulate in Benghazi, Libya, after ill-health forced her to pull out of this week's hearings.
Assistant Secretary Eric Boswell, head of the bureau of diplomatic security, had resigned after the release of the scathing report into the September 11 assault in which four Americans, including ambassador Chris Stevens, died.
The Accountability Review Board (ARB) had fingered four individuals for their "performance," State Department spokeswoman Victoria Nuland said in a statement.
Clinton "has accepted Eric Boswell's decision to resign as assistant secretary for diplomatic security, effective immediately," she said.
"The other three individuals have been relieved of their current duties. All four individuals have been placed on administrative leave pending further action."
She would not identify the three other State Department staff citing "personnel policies." But US television networks had earlier named one as Charlene Lamb, deputy assistant secretary for international programs.
"We have learned some very hard and painful lessons in Benghazi," said Deputy Secretary Bill Burns. He and Deputy Secretary Tom Nides are due to appear at public congressional hearings on Thursday.
The four were killed when dozens of Al-Qaeda linked armed militants stormed the consulate and attacked a nearby annex in an eight-hour fierce firefight.
The report revealed that repeated requests for increased security had been denied by staff in Washington and when the attack came they were overwhelmed.
"They did their best that they possibly could with what they had, but what they had was not enough," said ARB chairman veteran diplomat Thomas Pickering, praising US security staff in Benghazi for "their heroic efforts" that night.
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Burns said the department accepted all 29 recommendations in the 39-page report and admitted the investigation had taken "a clear-eyed look at serious, systemic problems, which are unacceptable."
But top Republican lawmaker Ileana Ros-Lehtinen said the panel made it clear that a lack of leadership and management "is to blame for the series of errors that resulted in the loss of life."
"The administration must continue to be held accountable," said Ros-Lehtinen, chairwoman of the House foreign affairs committee, insisting Clinton must "answer for these failures."
The report made public late Tuesday blamed "systemic failures and leadership and management deficiencies at senior levels" in the bureaus of diplomatic security and Near Eastern affairs for "grossly inadequate" security in Benghazi.
"Frankly, the State Department had not given Benghazi the security, both physical and personnel, resources it needed," Pickering told reporters, after briefing lawmakers early Wednesday on a classified section of the report.
Asked why the lessons learned from the twin embassy bombings in Africa in 1998 had failed to be fully applied, ARB vice chairman Admiral Mike Mullen said "the world has changed dramatically in this decade."
"I think we are in a much more difficult and challenging position with respect to meeting the needs to be out there and engage and doing so in a way that our people are very specifically secure."
Mullen also stressed that while Clinton has taken responsibility, as head of the department which deploys more than 60,000 people around the world, she had not been made aware of the specific security concerns in Benghazi.
Republican Senator Marco Rubio, who sits on the intelligence committee, insisted however Clinton's testimony was "indispensable to any effort to address this failure and put in place a process to ensure this never happens again."
The Benghazi attack also complicated President Barack Obama's plans for his second term cabinet, as his rumored top favorite to replace Clinton, the US envoy to the United Nations, Susan Rice, was forced to drop out of the running.
Rice had come under relentless Republican fire for saying, days after the assault, that, according to the best intelligence the available, it was triggered by a "spontaneous" protest outside the mission.
The ARB panel confirmed there had been no protest prior to the September 11 attack, nor was there any intelligence of a threat of any kind.