The chairman of the House committee investigating Hillary Clinton's use of a private email server said Thursday that the computer specialist tasked with setting it up "needs to come and testify."

Bryan Pagliano did not attend Tuesday's Oversight and Government Reform Committee hearing regarding the case, despite being supoenaed to do so. His attorneys said in a letter to Rep. Jason Chaffetz, R-Utah, that Pagliano would continue to assert his constitutional right not to testify.

"Subpoenas are not optional in this country," Chaffetz told Fox News' "The Kelly File" Thursday night. "You have to comply with them."

Pagliano previously told the FBI under immunity that there were no successful security breaches of the server. However, he said there had been failed login attempts that he described as "brute force attacks."

Last year. Pagliano refused to testify before the House Select Committee on Benghazi.

On Thursday, Chaffetz told Megyn Kelly that he had sent Pagliano a second subpoena asking him to produce the document granting Pagliano immunity and had received no response.

"This is one of the largest security breaches in the history of the United States State Department," Chaffetz said. "Of course we have questions for him."

When Kelly asked Chaffetz why Pagliano should be required to appear when he was most likely to plead the Fifth Amendment, Chaffetz replied, "Sometimes they talk, sometimes they plead the Fifth, but you have to provide that opportunity and we expect them to show up."

When challenged again by Fox News contributor and Democratic political consultant Julie Roginsky, Chaffetz said, "You don't actually know what they're going to do. I want to hear from him, himself."