Ahead of a highly anticipated Senate hearing later this week, FBI Director James Comey is being accused of making inconsistent statements about the bureau’s relationship with Christopher Steele, the ex-British spy behind the Trump dossier.

Iowa Sen. Chuck Grassley, the chairman of the Senate Judiciary Committee, made the allegation in a letter sent to Comey on Friday.

“There appear to be material inconsistencies between the description of the FBI’s relationship with Mr. Steele that you did provide in your briefing and information contained in Justice Department documents made available to the Committee only after the briefing,” Grassley wrote to Comey. (RELATED: FBI Director Pressed On Agreement With Trump Dossier Author)

Grassley will have a chance to quiz Comey on the issue on Wednesday, when the FBI chief testifies before the Judiciary Committee in a routine oversight hearing.

Grassley, a Republican, has repeatedly pressed Comey about reports that the FBI cooperated with Steele and even offered to pay the former MI6 agent $50,000 to continue his research on Trump. FBI agents reportedly met with Steele in July and again in October to discuss his findings, which were laid out in a 35-page dossier published by BuzzFeed in January.

Grassley has expressed concern about those discussions because Steele was working on behalf of an opposition research firm (Fusion GPS) that was, in turn, working for an ally of Hillary Clinton’s.

“The idea that the FBI and associates of the Clinton campaign would pay Mr. Steele to investigate the Republican nominee for President in the run-up to the election raises further questions about the FBI’s independence from politics, as well as the Obama administration’s use of law enforcement and intelligence agencies for political ends,” Grassley wrote in a March 6 letter to Comey.

Grassley says in his latest letter that he and California Sen. Dianne Feinstein, the top Democrat on the Judiciary Committee, were briefed by Comey on March 15. Attempts to find out from Grassley’s staff about what Comey said in that briefing that was inconsistent with Justice Department documents were unsuccessful.

“Whether those inconsistencies were honest mistakes or an attempt to downplay the actual extent of the FBI’s relationship with Mr. Steele, it is essential that the FBI fully answer all of the questions from the March 6 letter and provide all the requested documents in order to resolve these and related issues,” Grassley wrote.

Grassley also suggests that Comey’s failure to provide answers to questions about Steele could lead him to place holds on Justice Department nominees.

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It is unclear how important the dossier is to the FBI’s investigation. According to some reports, Comey has cited parts of the dossier in meetings with lawmakers. Other reports say that the FBI has corroborated some information in the dossier, which consists of 17 memos dating from June 20 to Dec. 13.

The FBI also cited the dossier in a September application for a surveillance warrant against former Trump campaign adviser Carter Page. Citing unnamed sources working in Russia, Steele wrote that Page met secretly with Kremlin officials in July to discuss ways to influence the election.

But the Trump campaign says that Page played a negligible role in the campaign and never spoke to Trump. Page has denied the claims about him in the document, which he refers to as the “dodgy dossier.”

Significant questions have been raised about other claims in the dossier. Several memos claim that Trump’s lawyer, Michael Cohen, met with Russian operatives in Prague in August. Cohen has said he was in the U.S. at the time the dossier claims he was at the meeting.

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