Senate Judiciary Chairman Chuck Grassley lambasted the FBI for what he called a "gag order" in preventing his panel from getting crucial information on oversight into fired FBI director James Comey.

"It's a serious threat to this oversight committee's authority," the Iowa Republican said at the beginning of Tuesday's hearing on the separation of power and two bills to protect special counsel Robert Mueller.

Grassley specifically mentioned what he called a "deal" made between the FBI and the Office of Special Counsel, an independent governmental watchdog agency that prosecutes violations of four specific federal statutes.

In August, Grassley and Sen. Lindsey Graham, R-S.C., said they obtained witness interview transcripts from OSC showing Comey had begun drafting a statement against the idea of filing criminal charges against Democratic presidential candidate Hillary Clinton over her private email server. That statement was allegedly drafted months before Clinton was interviewed in the FBI's probe into her private email server.

Grassley said Tuesday Senate Judiciary still wants to know if Comey had a "political motive" for drafting a statement — which would be a violation of the Hatch Act and something the office would prosecute.

The FBI "refused to cooperate voluntarily" with OSC's investigation into Comey, Grassley said Tuesday, so the two entered into what he called a "deal" as long as OSC promised "not to give any of the information to Congress."

"Nondisclosure agreements are essentially gag orders, plain and simple. They thwart transparency, accountability and seek to obstruct congressional oversight," Grassley said Tuesday, adding, "Executive branch agencies cannot limit our legislation power of inquiry merely by entering into nondisclosure agreements with one another."

Grassley continued: "Why was the FBI so focused on keeping Congress in the dark? Why is it so afraid of shining the light of day on the controversial decisions Mr. Comey made in the months before he was fired?"

Grassley reiterated his committee has key oversight responsibilities of the Department of Justice, and thus the FBI.

"If there's any whiff of inappropriate partisan or political influence in these institutions under any administration or any party, this committee has a duty to get to the bottom of it," Grassley said.

Grassley and Graham sent a letter to FBI Director Christopher Wray last month demanding all materials related to the OSC probe into Comey, as well as all of the FBI's records of what Comey drafted on Clinton — and Grassley said Tuesday he hopes the FBI will soon cooperate.

Ranking Member Dianne Feinstein, D-Calif., added that she has seen a piece of intelligence relevant to Senate Judiciary's oversight of the FBI that was given by OSC to the Senate Intelligence Committee only.

"Hopefully the [Senate] Intelligence Committee will be responsive with matters that involve our oversight," she said.