Thomas Pickering

The top Democrat on the House Oversight panel said Monday that the authors of an independent investigation into the deadly assault in Benghazi, Libya, should answer questions about their work at a congressional hearing, not in a private deposition that the Republicans want.

"If our committee is truly interested in improving the security of American diplomatic personnel overseas, members of our committee and the American public should hear first-hand from the individuals who have done the most exhaustive review of these attacks," Rep. Elijah Cummings of Maryland wrote in a letter to Rep. Darrell Issa, R-Calif., the panel's chairman.

In a Sunday talk show appearance, Issa said he would seek sworn testimony from veteran diplomat Thomas Pickering and retired Adm. Mike Mullen, the former chairman of the Joint Chiefs of Staff. The two conducted an independent investigation of the Sept. 11 attack that killed Ambassador Chris Stevens and three other Americans.

Their report was highly critical of the State Department's handling of at the U.S. outpost. Pickering, who also appeared on the Sunday shows, defended his scathing assessment but absolved former Secretary of State Hillary Rodham Clinton.

"We knew where the responsibility rested," said Pickering, whose career working for Republican and Democratic administrations, spans four decades.

Issa said he wants to know with whom the pair spoke to reach their conclusions about Clinton. Cummings suggested that they testify in public before the committee on May 22.

"This is a failure, it needs to be investigated. Our committee can investigate. Now, Ambassador Pickering, his people and he refused to come before our committee," Issa said Sunday.

Pickering, sitting next to Issa during an appearance on one Sunday show, disputed the chairman's account and said that he was willing to testify before the committee.

"That is not true," said the former top diplomat, referring to Issa's claim that he refused to appear before the committee.

In a separate interview, Pickering said he asked, via the White House, to appear at last Wednesday's hearing by the House Oversight and Government Reform Committee in which three State Department officials testified. He said he could have answered many of the questions lawmakers raised, such as whether U.S. military forces could have saved Americans had they dispatched F-16 jet fighters to the consulate, some 1,600 miles away from the nearest likely launching point.

"Mike Mullen, who was part of this report and indeed worked very closely with all of us and shared many of the responsibilities directly with me, made it very clear that his view as a former chairman of the Joint Chiefs of Staff that there were nothing within range that could have made a difference," Pickering said.

Republicans and Gregory Hicks, the former deputy chief of mission in Libya, have questioned why the military couldn't move faster to stop the two nighttime attacks over several hours. Hicks, who testified before the House Oversight panel, said a show of U.S. military force might have prevented the second attack on the CIA annex that killed security officers Tyrone Woods and Glen Doherty.

Robert Gates, a former Defense secretary, defended the decisions made at the time, saying: "I think my decisions would have been just as theirs were," adding "getting somebody there in a timely way - would have been very difficult, if not impossible."

The Accountability Review Board, which Pickering headed with Mullen, did not question Clinton at length about the attacks but concluded last December that the decisions about the consulate were made well below the secretary's level.

In her last formal testimony as secretary of State, Clinton appeared before two congressional committees investigating the Benghazi attacks. She took responsibility for the department's missteps and failures leading up to the assault, but said that requests for more security at the diplomatic mission in Benghazi didn't reach her desk.

Pickering and Mullen's blistering report found that "systematic failures and leadership and management deficiencies at senior levels" of the State Department meant that security was "inadequate for Benghazi and grossly inadequate to deal with the attack that took place."

Issa spoke on NBC's "Meet the Press." Pickering spoke on CNN's "State of the Union," CBS' "Face the Nation" and NBC. Gates appeared on CBS.