Glenn Simpson, the founder of opposition research firm Fusion GPS, will meet with the Senate Judiciary Committee on Tuesday to discuss his role in creating the uncorroborated anti-Trump dossier compiled by former British spy Christopher Steele.

Committee staffers plan to ask Simpson, a former Wall Street Journal reporter, about who hired his firm to conduct the Trump research.

Fusion’s clients have remained a mystery, even seven months after the dossier was published. According to various reports, a wealthy Republican hired Fusion in Sept. 2015 to investigate Trump. According to ABC News, the Republican paid Fusion GPS nearly $1 million.

As Trump ascended to a GOP primary win, the Republican donor dropped from the project, and Fusion found an ally of Hillary Clinton’s to maintain financing.

Rumors have floated around Washington about who paid Fusion to investigate Trump, but so far, Fusion has refused to disclose its clients’ identities.

Last June, Fusion hired Steele to continue the anti-Trump research. Steele, who is based in London, would go on to compile a dossier of salacious allegations about Trump and his campaign. The 17 memos in the dossier stretch the time period of June 20 to Dec. 13.

Iowa Sen. Chuck Grassley, the chairman of the Judiciary Committee, has had a deep interest in the dossier, mostly because of its importance to the ongoing investigations into whether the Trump campaign colluded with Russian government officials to influence the election.

The FBI used information from the dossier in its investigation, which began last July. FBI officials reportedly cited the dossier in an application to conduct surveillance on former Trump campaign adviser Carter Page.

FBI agents also reportedly met with Steele. In October, the two sides struck an informal deal to pay the former MI6 agent $50,000 to continue his investigation. Grassley has asked Fusion GPS and the FBI for details about that agreement, though it has been reported that the payment to Steele was never made.

It is unlikely that Simpson will provide answers to all of the Judiciary Committee’s questions. He has repeatedly dodged invitations and subpoena threats to appear before the committee. And in response to a document request submitted by the committee, Fusion GPS provided thousands of pages of nearly useless documents.

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