Story highlights

Two of Clinton's top State Department staffers are answering questions behind closed doors

A third is exercising his Fifth Amendment right to avoid self-incrimination

Washington CNN  — 

Hillary Clinton’s former chief of staff at the State Department testified Thursday to a congressional panel that none of Clinton’s work-related emails were withheld or destroyed, two sources familiar with the closed-door testimony told CNN.

Cheryl Mills, who oversaw the release of Clinton’s emails to the State Department, told the House Select Committee on Benghazi that no federal records related to the September 2012 terror attack on the U.S. mission were withheld or destroyed, the sources said.

Mills’ testimony came after Bryan Pagliano, an IT specialist who set up Clinton’s private email server, tried to fend off a subpoena by telling lawmakers that he will invoke his Fifth Amendment right against self-incrimination rather testify.

He is also refusing to cooperate with the FBI, Yahoo News reported. That could force the Justice Department to decide whether to offer him immunity – only possible in a criminal investigation.

Clinton’s top policy adviser, Jake Sullivan, who also staffed Clinton at the State Department, will also testify Friday before the House committee investigating the Benghazi attack.

Mills, who is believed to be the highest-profile Clinton aide interviewed by the House panel, spent more than nine hours in testimony.

She told the panel that Clinton and her staff were not trying to hide anything when they wiped her server, the sources said.

Clinton’s lawyer has insisted that only emails that were “personal” in nature were wiped.

Latest distraction for Clinton’s campaign

But as Mills testified on Thursday, Clinton’s campaign was marred with questions raised by Pagliano’s decision to invoke his right against self-incrimination.

Pagliano’s announcement could create a fresh opening for Clinton’s opponents in the 2016 presidential race to attack her, this time citing a former staffer’s actions rather than leveling their own charges, like Donald Trump’s suggestion that the controversy surrounding Clinton’s emails could force the end of her campaign.

Clinton spokesman Brian Fallon told CNN’s Brianna Keilar on Thursday that there’s nothing the campaign can do to change his mind.

“He’s not a part of the campaign. He’s a private citizen. And so, we can’t require him to do anything – he is represented by his own lawyer,” he said. “All we can do is give the same advice, recommendation and encouragement as Hillary Clinton has given all of her aides, current and former.”

Clinton’s aides had earlier sought to minimize the damage, saying they’d encouraged Pagliano to testify because he has nothing to hide.

RELATED: Former Clinton aide expected to plead the 5th

“We had hoped Bryan would also agree to answer any questions from the committee, and had recently encouraged him to grant the committee’s request for an interview,” a Clinton aide told CNN.

“Bryan is an utter professional and a wonderful young man who does not live in the public eye and understandably may not wish to be drawn into a political spectacle,” the aide said. “So his decision is both understandable and yet also disappointing to us, because we believe he has every reason to be transparent about his IT assistance.”

A Clinton aide said full cooperation has been encouraged with the Benghazi investigation.

“From the beginning, Hillary Clinton has continued to encourage cooperation even as the committee’s inquiry has ventured beyond the events at Benghazi, which had been the original scope of its mandate, to also include her use of a personal email account,” a Clinton aide said. “Already, several of Clinton’s senior-most aides from the State Department are scheduled to give interviews to the committee, as Clinton herself will do at an open hearing in October.”

Mills testified for more than nine hours

Even without Pagliano’s testimony, the House panel will hold several hearings connected to Clinton’s private email server.

Mills emerged from the more than nine hours of testimony to thank the committee for their “professionalism” and “respect” and “the work they’re doing.”

“The tragedy is more than what’s happening in this room,” she told reporters. “It was about the loss of individuals who were dear to the State Department and dear to this country. We honor them by remembering what happened, and doing our best to make sure that doesn’t happen again.”

Committee Chairman Rep. Trey Gowdy said Mills answered all of the committee’s questions and called the session “professional and fact-centric.”

While Clinton’s use of a private email address and server to conduct State Department business has not been a central focus of the panel, Gowdy has said the arrangement raises serious questions about whether the former secretary has handed over all of her Benghazi-related emails.

The committee’s top Democrat, Rep. Elijah Cummings, said Mills was forthcoming in her answers, and Mill thanked both men, telling reporters she would be taking her leave to go spend time with her family.

None of the three answered reporters’ questions.

Dispute over private hearings

Both Mills’ and Sullivan’s hearings are private – a decision made by Gowdy despite Mills’ request for hers to be made public amid fears that Republicans will selectively leak bits and pieces.

“We have not had a public interview with any other witness, and is there a myriad of reasons for that … not the least of which is, if you’re serious about getting the most amount of information, you’re going to do it in a transcribed interview session, not a public session,” Gowdy told reporters Thursday outside the hearing room.

Cummings disagreed with Gowdy’s decision and said he hoped that her entire transcript would be released immediately.

“This has turned into a derail Clinton mission,” he said just outside the hearing room. “$4 million in taxpayer money spent, and not looking at the issues we should be looking at.”