Senate Judiciary Committee Chairman Chuck Grassley is calling on the Justice Department to lift any gag order on a former FBI agent informant barring him from speaking to Congress about a Russian bribery scheme and its links to an Obama-era decision to approve a Moscow takeover of a U.S. uranium mine.

The Iowa Republican is investigating media reports that the U.S. government approved a Russian purchase of a large U.S. uranium mine amid an ongoing FBI investigation into bribes and money-laundering involving officials who worked for subsidiaries of the state-owned Russian firm seeking to acquire the uranium.

The Department of Justice during the Obama administration reportedly threatened to prosecute the informant if he discloses details of his involvement in the investigation to Congress.

Grassley said Thursday that the executive branch does not have the authority to use non-disclosure agreements to avoid congressional scrutiny. He demanded that Justice release the informant from any non-disclosure agreement and to provide him a copy of the agreement in question by Nov. 1.

"If the FBI is allowed to contract itself out of congressional oversight, it would seriously undermine our constitutional system of checks and balances," he said in a statement. "The Justice Department needs to work with the committee to ensure that witnesses are free to speak without fear, intimidation, or retaliation from law enforcement."

"Witnesses who want to talk to Congress should not be gagged and threatened with prosecution for talking," he said. "If that has happened senior DOJ leadership needs to fix it and release the witness from the gag order," he said.

The DOJ did not immediately respond to a request for comment.

Grassley said he has asked several agencies involved in approving the sale of the U.S. uranium mine to Russia whether they knew about the ongoing criminal investigation as well as "all communications" relating to donations to the Clinton Foundation, former President Bill Clinton's charity, by "interested parties in the transaction."

The agencies Grassley contacted include the Justice Department and State Department, he said. The senator, who has spent years writing legislation to protect government whistleblowers and informants, sent a letter to the Justice Department on Wednesday asking for a copy of any reported non-disclosure agreement and calling for the agency to lift it.

Additionally, he sent a letter to Victoria Toensing, the attorney representing the confidential informant, seeking an interview.

In the letter to Toensing, Grassley referred to an Oct. 18 report in the Hill indicating that the "informant's work was crucial to the government's ability to crack a multimillion dollar racketeering scheme by Russian nuclear officials on U.S. soil" and that the scheme involved "bribery, kickbacks, money laundering and extortion."

Quoting the Hill article, he also noted that the reporting indicates that "your client can testify that ‘FBI agents made comments to him suggesting political pressure was exerted during the Justice Department probe' and that ‘that there was specific evidence that could have scuttled approval of the Uranium One deal.'"

Grassley said the information the informant has appears to be "critical" to the Judiciary Committee's oversight of the Justice Department and "its ongoing inquiry into the manner in which" the U.S. government approved the uranium sale.

The Committee on Foreign Investment in the United States (CFIUS) approved a controversial deal to purchase a large U.S. uranium mine in 2010. The deal between Rosatam, the state-owned Russian company, and Canada-based Uranium One gave Moscow ownership of 20 percent of U.S. uranium.

Then-Secretary of State Hillary Clinton and then-Attorney General Eric Holder served on CFIUS at the time the agency approved the deal. She has said she knew nothing about the Russian racketeering.

Grassley, during a Judiciary Committee hearing Wednesday, said the Clinton Foundation had received millions of dollars from "interested parties" in the uranium deal and highlighted a $500,000 payment Bill Clinton received for a speech in Moscow before a Russian-government aligned bank.

That speech took place the same month the Russians began the process of acquiring the U.S. uranium mine amid a larger deal to acquire the umbrella company, Uranium One.

"This fact-pattern raises serious concerns about the improper influence on the process by the previous—by the Clintons during the Obama administration," Grassley asked Attorney General Jeff Sessions during the hearing.

"Has the Justice Department fully investigated whether the Russians compromised the Obama administration's decision to smooth the way for this transaction? And if not, why not?" he asked.

Sessions responded that it would be inappropriate to disclose whether the Justice Department is looking into the payments from the Clintons.

"Mr. Chairman, we are working hard to maintain discipline in the department—it wouldn't be appropriate for me to comment on any ongoing investigations," he said.

Sessions, minutes earlier, had assured Grassley that his concerns about the uranium deal would "be reviewed," although he declined to confirm or deny whether or not the Justice Department is actively investigating the matter.

"Mr. Chairman, we will hear your concerns. The Department of Justice will take such actions as is appropriate, I know," Sessions said, noting that "some people involved have gone to jail in that transaction already."