Fusion GPS, the opposition research firm behind the infamous Trump dossier, is again refusing to disclose the identity of political clients who financed the anti-Trump dirt-digging project.

In a letter to the Senate Judiciary Committee sent on Friday, lawyers for Fusion GPS said that disclosing the information about the firm’s opposition research project would undercut its First Amendment privileges and violate confidentiality agreements.

As has been widely reported, an unidentified Republican donor hired Fusion GPS in late 2015 to research Trump. But after the real estate magnate won the Republican primary, the donor dropped from the project. But Fusion GPS soon found another client, this one a Democratic ally of Hillary Clinton’s.

Fusion hired former British spy Christopher Steele to research Trump’s Russia connections. He produced a series of memos now known collectively as the dossier. The salacious document, much of which has been debunked, was leaked to numerous news outlets and published by BuzzFeed on Jan. 10.

Iowa Sen. Chuck Grassley, the chairman of the Judiciary committee, has taken an interest in the Democratic dossier donor’s identity, largely because the FBI used the document as part of the basis for its investigation into potential collusion between the Trump campaign and Russian government.

“When political opposition research becomes the basis for law enforcement or intelligence efforts, it raises substantial questions about the independence of law enforcement and intelligence from politics,” Grassley wrote in a March 24 letter to Glenn Simpson, a former Wall Street Journal reporter and owner of Fusion GPS.

Grassley has also pressed the FBI and Fusion to reveal details of their relationships with Steele. The bureau reportedly met with Steele multiple times to discuss the information in the dossier. In October, an informal deal was struck to pay Steele $50,000 to continue his research. The FBI reportedly ended up not making the payment, though it is unclear why.

One of Fusion’s arguments for not providing information to the Senate Judiciary Committee is that the FBI is likely to have the information being sought by the panel.

“The gravamen of your oversight investigation is whether the FBI complied with its own policy on working with paid informants,” reads Fusion’s letter. “The FBI can answer those questions, without forcing Fusion GPS to waive its constitutional and common law privileges and rights or those of its clients.”

This is the second time that Fusion GPS has refused to provide information to the committee. The Washington, D.C.-based company rebuffed the committee’s requests in April. The led to a subpoena threat from Grassley earlier this month.

In its latest letter, Fusion GPS says that it consulted with its political clients about the committee’s inquiry. But the unidentified financiers appear to not want their identity revealed.

Fusion GPS also appears to acknowledge that it has given the dossier to news organizations.

“In the June 7 letter, citing an article from The New York Times, you questioned whether waiver occurred ‘based on Fusion’s efforts to share the dossier with journalists and members of Congress.’ Without confirming or denying that story, releasing a report to a third party (including, but not limited to the government) would not cause a waiver of privilege,” Fusion’s lawyers said.

It is not entirely clear whether Fusion is BuzzFeed’s source for the dossier. But a reporter for BBC has acknowledged that Fusion gave him a copy of Steele’s memos prior to the Nov. 8 election.

Grassley has also pressed Fusion GPS over its work last year on behalf of a Russian businessman under investigation in the U.S. Fusion was hired by the law firm BakerHostetler, which had been hired by Prevezon Holdings, a Cyprus-based company which recently settled a money laundering case in the U.S.

Prevezon’s owner, Denis Katcyz, also hired a former Russian spy named Rinat Akhmetshin to lobby Congress to help roll back U.S. sanctions against Russia. (RELATED: Oppo Researcher Behind Trump Dossier Is Linked To Pro-Kremlin Lobbying Effort)

Correction: This article initially stated that Fusion GPS worked for Akhmetshin. The company instead worked for the law firm BakerHostetler, which worked last year for a lobbying client of Akhmetshin’s.

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