WASHINGTON (AP) — Aides to presidential candidate Hillary Rodham Clinton urged a former State Department employee who helped set up her private email server to testify before a House investigative panel, but the staffer has said he will assert his constitutional right not to testify.

A senior Clinton aide on Thursday called the response by Bryan Pagliano to a committee subpoena disappointing as the GOP-led panel investigates the deadly 2012 terrorist attack in Benghazi, Libya, and Clinton's use of a private email server during her tenure as secretary of state.

The aide was not authorized to publicly discuss private conversations and spoke only on condition of anonymity.

Attorneys for Pagliano sent the committee a letter Monday saying their client would not testify at a hearing planned for next week. The panel subpoenaed Pagliano last month.

Maryland Rep. Elijah Cummings, the top Democrat on the Benghazi panel, said he was not surprised that Pagliano would refuse to testify, given the "wild and unsubstantiated accusations" against Clinton.

The committee was launched last year to investigate the Obama administration's response to the Benghazi attacks that killed four Americans, including the U.S. ambassador. The investigation has widened in recent months to focus on Clinton's use of a private email account and server.

Clinton, the front-runner for the 2016 Democratic nomination for president, has dismissed both controversies as "partisan games."

Nick Merrill, a spokesman for Clinton's campaign, said in a statement Thursday that Clinton and her team "have been confident from the beginning that Clinton's use of a personal email was allowed and that she did not send or receive anything marked classified, facts confirmed by the State Department and the inspector general" for the department.

Clinton "has made every effort to answer questions and be as helpful as possible, and has encouraged her aides, current and former, to do the same, including Bryan Pagliano," Merrill said.

Clinton is set to testify before the Benghazi committee next month.

Meeting behind closed doors on Thursday, members of the House panel were questioning Cheryl Mills, Clinton's former chief of staff. Jake Sullivan, another former top aide who now works on Clinton's presidential campaign, was set to be interviewed on Friday. Both sessions will be off limits to the public.

Republican Rep. Trey Gowdy of South Carolina, chairman of the Benghazi committee, said before the interview began that Mills "is no different from any other witnesses" who also have been interviewed in private.

The panel was likely to ask Mills about her role in preparing "talking points" for administration officials following the attack on U.S. diplomatic facilities in Benghazi.