Story highlights Hill source tell CNN a top Hillary Clinton aide had a narrow immunity deal during an FBI's investigation

The investigation stems from Clinton's use of a private email server while she was secretary of state

Washington (CNN) Hillary Clinton's top aide was given a limited immunity deal by the FBI in the investigation into Clinton's private email server, according to congressional sources Friday.

Cheryl Mills, Clinton's chief of staff and adviser at the State Department, was given the narrow immunity deal during the FBI's investigation into the former secretary of state's use of a private email server during her time in office.

"This is beyond explanation," Rep. Jason Chaffetz said in a statement. "The FBI was handing out immunity agreements like candy. I've lost confidence in this investigation and I question the genuine effort in which it was carried out. Immunity deals should not be a requirement for cooperating with the FBI."

A spokeswoman for Chaffetz, chairman of the House Oversight Committee, confirmed that Mills was being given immunity. But a Democratic congressional source said the extent of the deal was limited to a search of her and another aide's laptop.

According to the source, the FBI requested the laptops of Clinton's attorneys, Mills and Heather Samuelson, after they conducted a 2014 review of what emails Clinton sent were work-related and which were personal. Work-related emails were turned over to the State Department and personal emails were deleted.

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