Senate Judiciary Committee Chairman Chuck Grassley is threatening to subpoena two top FBI officials to answer questions about Russia, the firing of James Comey, the Hillary Clinton email case and more.

“If we are unable to reach a voluntary agreement, the committee will consider proceeding to compulsory process through its authority under the Constitution and pursuant to Senate and committee rules,” Grassley wrote in a Wednesday letter to the Justice Department.

Grassley, an Iowa Republican, has been seeking a closed-door interview with FBI officials Carl Ghattas and James Rybicki. Ghattas is the executive assistant director of the FBI National Security Branch; Rybicki is the the chief of staff and senior counselor to the FBI director.

Stephen Boyd, the assistant attorney general, had written in a Sept. 11 letter to Grassley that the officials could not be interviewed over concerns it would interfere with the ongoing special counsel probe into Russian meddling in the 2016 election.

But Grassley said he wants to ask the officials about improper political influence or bias in the Justice Department, Comey’s firing, the Clinton email investigation, the FBI’s activities related to President Trump and his associates and Russia’s interference in the 2016 election.

DOJ REFUSING TO LET FBI STAFFERS SPEAK TO SENATE INVESTIGATORS

One of those officials, Rybicki, has been in the spotlight lately after partial interview transcripts emerged in which Rybicki and another bureau official suggested Comey had started drafting an exoneration statement for Clinton in the private email investigation weeks before she was interviewed.

The claims, which emerged in interviews by the Office of Special Counsel and were disclosed by members of the Judiciary Committee, prompted some GOP lawmakers to question Comey’s past statements to Congress. On the other side of Capitol Hill, some House Republicans have renewed calls for a second special counsel to be appointed in light of claims Comey drafted an exoneration statement before the case was closed.

Clinton, the Democratic presidential nominee in 2016, was investigated by the FBI for using a private email address and server to handle classified information while serving as secretary of state.

In July 2016, Comey famously called Clinton’s email arrangement “extremely careless” though he decided against recommending criminal charges.

Comey was fired as FBI director by President Trump in May amid tensions over the Russia investigation.

While the Department of Justice has turned down Grassley's requests for access to the officials in question, Fox News has learned Deputy Attorney General Rod Rosenstein met with Grassley recently for a private face-to-face meeting.

The Senate Judiciary Committee originally asked in July to interview Rybicki and Ghattas.

Fox News’ Jake Gibson and Chad Pergram contributed to this report.