Updated

The Hillary Clinton campaign on Sunday defended the Justice Department's decision to grant immunity to Cheryl Mills and other Clinton aides in the FBI’s investigation into Clinton's use of a private server system while secretary of state.

Clinton campaign strategist Joel Benenson told “Fox News Sunday” that such deals are “fairly routine” and pressed the point that Mills received only “limited immunity.”

Revelations about Mills, a chief of staff for Clinton at the State Department, and two other staffers being granted immunity as part of the FBI probe emerged last week in an Associated Press interview with House oversight committee Chairman Jason Chaffetz, R-Utah.

Benenson said such immunity is granted to encourage witnesses to cooperate by ensuring them that anything uncovered in their files outside the scope of the investigation cannot be used against them.

In Mills’ case, she turned over a laptop computer, and her testimony in the FBI investigation and potential testimony before Congress was not covered in the immunity deal.

“So a limited immunity is when anything else is irrelevant to them,” Benenson said Sunday. “They wanted to make sure they had access to Cheryl Mills' information about e-mails -- not anything else.”

Mills and four other Clinton aides have been granted some form of immunity, including lawyer Heather Samuelson, then-State Department information resources Director John Bentel and former agency IT specialist Bryan Pagliano.

The immunity issue -- as an example of why voters doubt Clinton’s trustworthiness -- could come up Monday night during the first presidential debate between Clinton, the Democratic presidential nominee, and Republican rival Donald Trump, Kellyanne Conway, Trump’s campaign manager, told ABC's "This Week."

The agency’s two-year investigation found that several Clinton emails via the server system included at least parts of classified information and that Clinton was “extremely careless” with that information. However, the FBI concluded the investigation without recommending criminal prosecution.

“Immunity deals should not be a requirement for cooperating with the FBI,” Chaffetz said Friday in a statement.

Clinton campaign spokesman Brian Fallon said Republicans were just “trying to make something out of nothing by rummaging through the files of a Justice Department investigation that was closed months ago without any charges whatsoever.”

The Associated Press contributed to this report.