A senior member of Hillary Rodham Clinton's inner circle testified Friday before a House panel investigating the deadly 2012 attacks in Libya as a nasty spat erupted between a Republican staffer and a Democratic lawmaker who insists it's time for the committee to disband.

Jake Sullivan, a former policy director and deputy chief of staff under Clinton at the State Department, was questioned by the panel in 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.

Maryland Rep. Elijah Cummings, the senior Democrat on the committee, said at midday that Sullivan had answered every question.

The panel is investigating the Sept. 11, 2012 attacks at the U.S. facilities in Benghazi that killed four Americans, including U.S. Ambassador Chris Stevens.

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

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

He said the panel had let down the families of those killed in the attacks.

That prompted a highly unusual attack from Jamal Ware, a spokesman for the GOP-led panel. In an email to reporters, Ware blasted Schiff, saying he had not attended enough of the panel's meetings to levy criticism, including Friday's session. Schiff was traveling from California to Washington on Friday.

"You all need to ask Mr. Schiff how it is he has drawn these conclusions since he has only seen fit to attend one hour of one witness interview since the committee was constituted," Ware said.

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.

Clinton acknowledged on Friday that the use of a private email system wasn't the "best choice" and acknowledged she didn't "stop and think" about the set-up when she became secretary of state in 2009. In an interview with NBC News, Clinton did not apologize for her decision. She said she wishes she had "made a different choice" and takes responsibility for the decision.

The GOP-led House panel that was established to investigate Benghazi has gotten immersed in the controversy surrounding Clinton's email practices.

Cheryl Mills, Clinton's former chief of staff, answered questions for the panel for 9 1/2 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 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.

In his op-ed, Schiff called for the panel to be shut down.

"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 wrote.

Cummings 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.

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