WASHINGTON — Three witnesses ordered to testify Tuesday before a House committee investigating Hillary Clinton's use of a private email server asserted their constitutional rights against self-incrimination and did not appear or refused to answer questions.

Bryan Pagliano, the former State Department computer specialist tasked with setting up Clinton's server, did not attend the Republican-led hearing. His attorney said in a letter to the chairman of the Oversight and Government Reform Committee that Pagliano will continue to assert his constitutional right not to testify.

Pagliano spoke previously to the FBI under immunity, telling the bureau there were no successful security breaches of the server. But he said he was aware of many failed login attempts that he described as "brute force attacks."

Pagliano also refused to answer questions last year before a House panel investigating the deadly 2012 attacks in Benghazi, Libya.

The email issue has shadowed Clinton's candidacy, and Republicans have been steadfast in focusing on her use of a private server for government business, with several high-profile hearings leading up to the election. Democrats insist the sole purpose of the hearings is to undermine Clinton's bid for the presidency.

"I believe this committee is abusing taxpayer dollars and the authority of Congress in an astonishing onslaught of political attacks to damage Secretary Clinton's campaign for president," said Rep. Elijah Cummings of Maryland, the committee's top Democrat.

Two officials from Denver-based Platte River Networks appeared before the committee but invoked their constitutional right not to testify. Bill Thornton and Paul Combetta were excused from the session. In June 2013, after Clinton had left office, the server was moved from her Chappaqua, New York, home to a data center in northern New Jersey, where it was maintained by the Platte River Networks.

One witness, Justin Cooper, a former White House aide to President Bill Clinton, is answering the committee's questions. Chaffetz said Cooper purchased the first server used by Clinton and registered the clintonemail.com domain name. Cooper also helped set up Clinton's mobile communications.

Cooper told the committee that he did not have a security clearance during the period he was performing this work.

Congressional Republicans last month issued subpoenas to Platte River Networks and two other companies — Datto Inc. and SECNAP Network Security Corp. — after they declined to voluntarily answer questions to determine whether Clinton's private server met government standards for record-keeping and security.

The committee's chairman, Rep. Jason Chaffetz, R-Utah, said there will be consequences for Pagliano's refusal to appear and for "thumbing his nose at Congress."

He didn't specify what the penalties would be but said, "We're not letting go of this."