Senate Judiciary Committee Chairman Chuck Grassley Charles (Chuck) Ernest GrassleyGardner signals support for taking up Supreme Court nominee this year Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee McConnell digs in on vow to fill Ginsburg's Supreme Court seat MORE (R-Iowa) on Tuesday slammed the Department of Justice and FBI for not complying with requests for two FBI agents to testify before the panel.

Grassley said that the FBI's claim that nondisclosure agreements prevented testimony at a hearing on codifying the special counsel in the Russia investigation was not valid and not permitted under the Constitution. He referred to the agreements as attempts at de facto "gag orders."

"Why is the FBI so focused on keeping Congress in the dark?" he asked.

"Why is it so afraid of shining the light of day on the controversial decisions of Mr. Comey in the months before he was fired?" he added, referring to former FBI Director James Comey.

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Grassley wants the FBI's national security division head, Carl Ghattas, and Comey's former chief of staff, James Rybicki, to discuss the final days of Comey's tenure, including the investigation into former Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE's use of a private email server while secretary of State. He has threatened to subpoena the agents if the FBI does not cooperate.

After his comments, the ranking Democrat on the Senate Judiciary Committee, Diane Feinstein (Calif.), raised similar issues at the hearing regarding the CIA.

Feinstein said that she largely agreed with Grassley's comments, adding that the same arguments should also apply to the CIA. She claimed the agency had intelligence relevant to the Judiciary panel's investigations that she had come across through her work in the Intelligence Committee.

"It seemed to me that when I saw it, it was relevant to this committee’s oversight. Yet this committee cannot receive it," she said.

Heavily redacted documents released by the Justice Department to the committee show that Comey began to prepare his comments dismissing charges against Clinton before interviewing several witnesses. Due to the redactions, Grassley has said he is not confident he does not have the entire story.

The department has said that allowing FBI agents to testify might interfere with the special counsel's investigation into Russian meddling in last year's presidential election.