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Cheryl Mills, right, a longtime aide to Hillary Rodham Clinton, arrived for an interview before the House Benghazi committee on Capitol Hill on Thursday.Credit Alex Wong/Getty Images

WASHINGTON — Cheryl Mills, a longtime lawyer and aide for Hillary Rodham Clinton, walked into a Capitol Hill conference room shortly before 8 a.m. on Thursday for an interview under oath about Mrs. Clinton’s email account and the 2012 attacks in Benghazi, Libya. As Ms. Mills entered the room, she declined to talk to reporters who asked her whether she would answer questions about Mrs. Clinton’s server and whether there was classified information on it.

The interview before the House Benghazi committee is expected to last several hours.

The committee’s chairman, Trey Gowdy, Republican of South Carolina, did address reporters for a few minutes before the hearing, saying he had rejected Ms. Mills’ request to hold the interview in public because the committee has conducted its other interviews in private.

“If you’re serious about getting the most amount of information, you are going to get it in a transcribed interview session, not in a public session where members are limited to five or 10 minutes,” Mr. Gowdy said. “So we have not had a public fact finding interview with a single witness, and I’m not going to treat her any differently.”

On Wednesday it was revealed that Byran Pagliano, a former information technology official at the State Department who helped set up the server that housed Mrs. Clinton’s private email account, plans to invoke his Fifth Amendment right in response to questions from the committee. Mr. Pagliano’s lawyer declined to comment.

A Clinton campaign aide said that Mr. Pagliano’s decision was “both understandable and disappointing to us, because we believe he has every reason to be transparent about his I.T. assistance.”

“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,” the aide said.

The aide added, “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.”

Mr. Gowdy addressed the issue briefly before stepping into the interview. “You’re free to claim whatever inference you want from the fact that” he does not want to testify, Mr. Gowdy said.