House Republicans on Thursday attacked the integrity and findings of the independent review board which probed the Benghazi consulate terror attack and questioned if auditors protected higher-ups at the State Department.

House Oversight Committee Chairman Darrell Issa, R-Calif., clashed with the authors of the Accountability Review Board (ARB) investigation, former Ambassador Thomas Pickering and Admiral Michael Mullen, during a contentious hearing on Capitol Hill.

Issa, following the charges made in a report his committee released earlier this week, accused the ARB of downplaying security decisions made by senior officials at the State Department, especially those of Under Secretary Patrick Kennedy, while blaming four subordinates for failing to provide enough security for the Benghazi post.

He also questioned the ARB's decision not to interview senior State Department officials, including then-Secretary of State Hillary Clinton and two of her deputies, and suggested that the board had a conflict of interest in relying so heavily on the State Department for staff and resources. Republican lawmakers were particularly troubled that Kennedy supervised the selection of the Benghazi ARB staff, and State appointed four of the five members of the board.

During a particularly testy exchange, Rep. Jim Jordan, R-Ohio, questioned why Mullen reached out to give Cheryl Mills, Clinton's chief of staff, a “heads up” before Charlene Lamb, deputy assistant secretary for international programs, testified before Congress.

Mullen told Mills that he thought that Lamb's appearance could be “difficult” for the State Department, according to the Oversight Committee's report.

“If an inspector general working on an investigation of the military had done that, would you believe they had a problem doing their jobs?” Jordan asked.

Mullen didn't answer because Jordan's time for questioning was up. Just seconds before, he testified that he briefed both Mills and Clinton on the report before it was finalized.

Republicans also continued to grill Pickering and Mullen about why military assets in the region were not ready and mobilized to even attempt a rescue of the Americans under attack at the diplomatic post.

Mullen stood by his previous assessment that the military did not have aircraft or other assets ready to deploy to Benghazi, estimating that it would have taken 10 to 20 hours to get them ready.

“It became a physics problem,” he said.

Rep. Jason Chaffetz, R-Utah, read a list of names of special forces commanders and others in the chain of command overseeing the Africa region at the time of the Benghazi attack. Mullen testified that he had not interviewed several mentioned by Chaffetz, but had questioned former Gen. Carter Ham, the chief of U.S. Africa command.

Several Democrats, led by Rep. Elijah Cummings, D-Md., defended Pickering and Mullen, as well as their review, calling them widely respected public servants who have a combined 83 years of experience at the State Department and in the military among them.

The Benghazi report, Cummings said, was one of the most comprehensive probes in history, having interviewed more than 100 people and reviewed thousands of pages of documents.

“Some say it doesn't answer any real questions and its sole function was to insulate Hillary Clinton,” he said during the hearing. “Those kinds of statements upset me because I think they are so unfair and we're better than that.”

At the beginning of the hearing, Issa read a letter provided by the family of murdered Libyan ambassador Christopher Stevens that described how Stevens would not have wanted his death politicized.

The letter said Stevens understood the security risks his job entailed and still traveled the country “with the lightest of escorts,” sitting in open cafes, getting to know the Libyan people and earn their respect and admiration.

“He knew there were risks to that kind of accessibility, and he accepted those risks,” Issa read from the letter.

Republicans planned to use the second half of the hearing to question why Obama administration officials initially blamed the attack on a spontaneous response to an anti-Islam Internet video even as diplomats in Libya said during the assault that it was a pre-planned terrorist attack.

Pat Smith, the mother of State Department employee Sean Smith, who died in the attack, handed out prepared remarks to reporters before the hearing.

President Obama and other senior U.S. officials, she said, “looked me in the eyes” and told her the attack happened because of the video and “they would keep me advised,” she said.

“I was told I would be invited to a medal ceremony; they would let me know when. I did not hear from them again.”

Smith also said she still has questions about why the facility did not have sufficient security “when people on the ground were begging for it.”