By Jamie Crawford

The leader of a review board that investigated the deadly terror attack on the U.S. diplomatic post in Benghazi, Libya, lacked sufficient independence to reach an objective finding of fault, a congressional committee chairman said on Thursday.

House Oversight Committee Chairman Darrell Issa told retired Ambassador Thomas Pickering that he failed to see how the Accountability Review Board could have come to an objective conclusion about the September 2012 attack based on Pickering's long career as a State Department official.

"You talked about 42 years in the organization you were overseeing," Issa said to Pickering, who drew on his diplomatic experience to help him lead the panel.

"If we looked at the bank failures of 2007 and brought Jamie Dimon in to head the board, some might say that there was an inherent conflict because of his experience in life," Issa said of the JPMorgan Chase chairman.

"Mr. chairman, with greatest respect, this was not, quote, a 'gotcha' investigative panel," Pickering replied.

He asked why a group looking for answers would be empaneled without understanding the specific minutiae of how diplomacy is carried out.

"I appreciate that," Issa shot back. "Obviously, this was not a 'gotcha' panel, because nobody was 'gotcha-ed.'"

The exchange between Issa and Pickering illustrated the sharp political emotion that has defined many exchanges over the Benghazi attack by armed militants, which killed Ambassador Christopher Stevens and three other Americans.

Issa's investigation has been a partisan flashpoint as he has pushed the Obama administration hard to get a better understanding of pre-attack security at the diplomatic outpost and why no one at the State Department lost their jobs after Pickering's investigation noted shortcomings.

Earlier this week, the Republican majority staff of the committee released a report that also raised new questions. It noted the relatively short time it took the review board to investigate the attack and issue its findings, and pointed out that those interviewed by the panel were not made available to members of Congress.

The Democratic minority staff, led by its ranking member Rep. Elijah Cummings of Maryland, released its own report just as Thursday's hearing got underway. It questioned the findings of the Republican staff report.

"Based on all of the evidence obtained by this committee, this Benghazi review was one of the most comprehensive ARB reviews ever conducted," Cummings said. "I have seen no evidence, none whatsoever, to support these reckless Republican accusations. To the contrary, witness after witness told the committee that the ARB's work was 'penetrating,' 'specific,' 'critical,' 'very tough,' and the 'opposite of a whitewash.'"

But the fireworks were just getting started as a session that ran more than four hours got underway. It examined numerous areas around how and why certain facets of the review board investigation were undertaken.

Former Joint Chiefs Chairman retired Adm. Michael Mullen, who served as Pickering's co-chair, was effusive in his assertions that there were no orders for any military detachments to "stand down" that had already put in motion to try and arrest the assault on the diplomatic building and a nearby CIA annex.

"This is not something you can just wish to happen instantly. There's a lot of planning, preparation, as rapidly - to do it as rapidly as one can do it," Mullen said in reference to questions of how no U.S. military assets made it to Benghazi that night.

"We are not big enough in the military to be everywhere around the world to respond to where every embassy is that might be high-risk. We have to take risks and figure that out," he said.

Questions emerged from multiple members of the committee as to why the review board did not assign any culpability for management and other shortcomings to former Secretary of State Hillary Clinton as head of the department.

"We had very clear evidence, full and complete to our information, that the authority - responsibility, the accountability rested with the people we identified," Pickering said in explaining why failures in the State Department structure were centered at the assistant secretary level.

"If the secretary (Clinton) wasn't involved, I must be on another planet," Rep. John Mica said in response.

In interviews with media outlets prior to her stepping down earlier this year, Clinton said she took responsibility for the security of diplomats and diplomatic outposts around the world in her role as secretary.

There have been 18 such review boards since 1986 that have investigated attacks on U.S. facilities overseas.

Many members of the panel questioned Pickering and Mullen as to why certain recommendations from the report that looked into the 1998 bombings of U.S. embassies in East Africa were not carried forward.

"Secretary (Madeleine) Albright as a result of that recommendation, met daily with the assistant secretary of state for diplomatic security first thing in the morning. And that established a nexus, a chain, which neither her - I think none of her three successors kept. I think that may have been an error," Pickering said.

"I think that in some ways her interest - and put it this way - in no more Nairobis and no more Dar es Salaams was an important instinct. I think that that was a rather good process, and in some ways I'm sorry it wasn't repeated," he said.

The review board led by Pickering and Mullen made 29 recommendations, one of which was to establish another independent review to identify "best practices" in the public and private sectors in security intelligence, risk management and accountability - all areas where problems were identified at the State Department.

That panel, led by former Secret Service Director Mark Sullivan, warned that the State Department did not pay enough attention to the bureau overseeing security for 275 diplomatic posts and called for it to be elevated in its importance to deal with a growing threat.

As a result of the Benghazi attack, the State Department created a new position of deputy assistant secretary of state for high threat posts and has begun to beef up security and improve training.

But Issa contended Thursday that it was the purview of his committee and that of Congress to interview many of the same witnesses who were on the ground in Benghazi that spoke to the review board in order to get an understanding of where accountability for the attack lay.

"I am in the process of issuing subpoenas because the State Department has not made those people available, has played hide and go seek, and is now hiding behind a thinly veiled statement that there's a criminal investigation," he said of the FBI probe.

And in the next sentence, Mr. Issa laid out the roadmap for his committee in the Benghazi investigation.

"That's part of the reason that this investigation cannot end until the State Department gives us at least the same access that they gave your board," he said.

CNN's Elise Labott contributed to this report.