WASHINGTON (AP) — The chairman of a congressional panel investigating the deadly Benghazi attacks said Friday that a member of Hillary Rodham Clinton's inner circle could provide key insight into how the State Department maintained its presence in Libya prior to the 2012 attack.

Jake Sullivan, a former policy director and deputy chief of staff under Clinton at the State Department, arrived early Friday for what is expected to be a daylong session of testimony behind closed doors.

Rep. Trey Gowdy, R-S.C., the committee's chairman, said Sullivan was in a "unique position" to talk about how U.S. policy in Libya required the State Department to have a physical presence in the country. Sullivan is currently a top policy aide on Clinton's presidential campaign.

Mike Morrell, the CIA's former deputy director, likely will be the next witness to appear before the panel, Gowdy told reporters.

Clinton, the Democratic front-runner for the 2016 nomination, has been dogged by criticism about her use of a private email server for government business during her tenure as secretary of state, and she has struggled to explain her decision.

The panel that was established to investigate Benghazi has gotten immersed, as well, in the controversy surrounding Clinton's email proclivities.

Cheryl Mills, Clinton's former chief of staff, answered questions for the panel for 9½ hours on Thursday. Few details were released, but knowledgeable officials said lawmakers asked Mills about her role in preparing "talking points" for administration officials following the 2012 attack on U.S. diplomatic facilities in Benghazi that killed four Americans, including the U.S. ambassador to Libya.

Gowdy said Friday that Mills answered all of their questions in a professional manner and that inquiries which focused on a private email server Clinton used while serving as secretary of state, including its set-up, came only later in the session.

"Our committee is the committee on Benghazi. It's not the committee on emails," Gowdy said.

Democrats called on Republicans to disband the committee. Rep. Adam Schiff, D-Calif., a member of the panel, wrote in an op-ed in The New York Times that it had become "little more than a partisan tool to influence the presidential race, a dangerous precedent that will haunt Congress for decades."

"Whatever their original purpose, the Select Committee's leaders appear no longer to have any interest in Benghazi, except as the tragic events of that day may be used as a cudgel against the likely Democratic nominee for president," he said.

Mills, a lawyer who has worked for former President Bill Clinton, said after Thursday's meeting that she was treated with professional courtesy and respect.

"Ultimately the tragedy in Benghazi was about the loss of (four) individuals dear to the State Department and dear to this country," Mills said. "We honor them by remembering what happened and doing our best to make sure it doesn't happen again."

Gowdy said Mills' session would be treated as classified and declined to answer any specific questions.

Maryland Rep. Elijah Cummings, the senior Democrat on the committee, called for Gowdy and committee Republicans to release a transcript of Mills' testimony as soon as possible, so the public can learn what questions were asked and how they were answered.

"I don't like the idea of putting out a little bit of information so that folks can write headlines, and then trying to find the facts to correct them," Cummings said Friday.

The request to release the transcript was unlikely to be granted, Gowdy said, noting that much of what was discussed related to national security.

The interviews with Mills and Sullivan have been upstaged by a lesser-known character in the Benghazi saga, a former State Department employee who helped set up Clinton's private email server. The former staffer, Bryan Pagliano, told the committee earlier this week that he will assert his constitutional right not to testify at a meeting set for next week.

Pagliano was a State Department employee from 2009 to 2013 and is now a private contractor working in the department's Bureau of Information Resource Management, according to a department official who asked not to be identified when discussing personnel matters.

Pagliano's response to a committee subpoena was unwelcome news to Clinton aides who had pressed him to be interviewed by the GOP-led panel.

Cummings said he was not surprised that Pagliano would refuse to testify, given the "wild and unsubstantiated accusations" against Clinton.

"This investigation has turned into a (quest to) derail Hillary Clinton's nomination by any means necessary," Cummings said.

Clinton has dismissed the controversy as "partisan games." She also has said she regrets using a personal email account to conduct government business.