"I am launching an accountability review board that will be chaired by Ambassador Thomas Pickering," Clinton told reporters in Washington.
Pickering, a retired diplomat who holds the highest rank in U.S. foreign service, is a former U.S. undersecretary of state who was also ambassador to Russia, India, the United Nations, Israel, El Salvador, Nigeria and Jordan.
Time magazine called him a "five-star general of the diplomatic corps" when he was appointed undersecretary of state in 1997.
The review board is to investigate the Benghazi rocket-propelled grenade attack on the consulate on the 11th anniversary of the Sept. 11, 2001, attacks, including the level of security in place at the time and whether security failures might have contributed to the killings of Stevens and three other State Department officials in the attack, officials said.
The three other officials were computer expert Sean Smith and two former Navy SEALs, Greg Doherty and Tyrone Woods, who were working as consulate security contractors.
The assault was the sixth by militants in Benghazi since April, including a bombing June 6.
The review board's investigation and report will be separate from an FBI probe of the attack, Clinton said.
Clinton spoke before briefing House and Senate lawmakers in separate closed-door sessions on Capitol Hill. She was joined by Director of National Intelligence James Clapper and other senior officials.
A House subcommittee chairman separately told Clinton his panel would conduct its own investigation because of discrepancies and uncertainty in the accounting of what happened.
"The universe of known facts ... remains small, and confusion has overshadowed certainty in this matter," Subcommittee on National Security, Homeland Defense and Foreign Operations Chairman Jason Chaffetz, R-Utah, said in a letter to Clinton.
"The American people have a right to know precisely what happened," he said.
He gave Clinton a 5 p.m. EDT Oct. 4 deadline to provide the subcommittee with all classified and unclassified analyses and other assessments of Benghazi consulate security, preliminary FBI findings and other materials, including documents the administration used to make public assertions the attack appeared not to be a long-planned operation but rather a spontaneous assault after word circulated of an Internet video considered offensive to Islam.
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