U.S. District Court Judge Royce Lamberth said he was "shocked" and "dumbfounded" when he learned that FBI had granted immunity to former Clinton chief of staff Cheryl Mills during its investigation into the use of Clinton's server, according to a court transcript of his remarks.

"I had myself found that Cheryl Mills had committed perjury and lied under oath in a published opinion I had issued in a Judicial Watch case where I found her unworthy of belief, and I was quite shocked to find out she had been given immunity in ‐ by the Justice Department in the Hillary Clinton email case," Lamberth said during Friday's hearing.

The Department of Justice's Inspector General (IG), Michael Horowitz, noted in a bombshell report in June that it was "inconsistent with typical investigative strategy" for the FBI to allow Mills to sit in during the agency's interview of Clinton during the email probe, given that classified information traveled through Mills' personal email account. "[T]here are serious potential ramifications when one witness attends another witness' interview," the IG wrote.

On Friday, Lamberth, who was appointed to the bench by President Ronald Reagan, said he did not know Mills had been granted immunity until he "read the IG report and learned that and that she had accompanied [Clinton] to her interview."