WASHINGTON — The head of the House Oversight and Government Reform Committee has subpoenaed the co-chairman of the independent review board that investigated last year's attack on the U.S. diplomatic mission in Benghazi, Libya, to answer questions about the panel's findings behind closed doors.

Rep. Darrell Issa, R-Calif., said in a statement Friday that he had issued the subpoena to retired veteran diplomat Thomas Pickering to force him to appear at a deposition next week. Pickering, who co-chaired the Benghazi Accountability Review Board with a former Joint Chiefs of Staff chief Mike Mullen, has offered to testify before Issa's committee in public. But Issa said a closed-door meeting is needed first in order for the committee to fully understand how the review board conducted its investigation.

"While I am very much committed to having you testify publicly and appreciate your newfound willingness to do so, I was disappointed that you are attempting to limit the committee's understanding of the Accountability Review Board by refusing to participate in a voluntary transcribed interview prior to testifying publicly," Issa said in a letter to Pickering. "In light of your continuing refusal to appear voluntarily for a transcribed interview, however, I have found it necessary to issue a subpoena to compel your appearance at a deposition."

Issa's letter, which was released by his office, said he would consider lifting the subpoena for next Thursday's deposition if Pickering agreed to show up on his own.

At an event at the Middle East Institute Friday, Pickering said he had just learned of the subpoena and declined immediate comment.

"I've just heard about it and obviously will want to talk to my friends at the State Department and seek legal advice and all the other things that go with it," he said.

Issa complained that prior to a public hearing about Benghazi that he chaired last week, Pickering had refused to speak with him and other members of the committee.

Pickering said Sunday that he had sent a request through the White House to testify at Wednesday's hearing, but, "we were told the majority said I was not welcome at that hearing. I could come at some other time." He made his comment on NBC's "Meet the Press."

Issa is one of several GOP lawmakers who have suggested the Obama administration is trying to cover up the circumstances and aftermath of the Sept. 11, 2012, attack on the Benghazi outpost that killed the U.S. ambassador to Libya and three other Americans.

The review board convened by then-Secretary of State Hillary Rodham Clinton was harshly critical of the State Department, blaming systematic leadership and management failures at senior levels for inadequate security in Benghazi. It made 29 recommendations to improve matters, and the State Department has vowed to implement all of them.

Issa said numerous questions about the review board's report remain unanswered, including its methodology. He noted that the ARB conducted its work in secret and appears not to have recorded or transcribed its interviews with witnesses.

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