The co-founder of a firm behind the controversial opposition research dossier on President Trump spoke to investigators on the Senate Judiciary Committee on Tuesday behind closed doors.

ABC News reported that Glenn Simpson was set to be questioned about funding and sources for the dossier, which contained salacious and unverified information about the president.

Simpson's firm, Fusion GPS, was first hired by Republicans to dig up dirt on Trump during his presidential campaign. But as Trump emerged as the party's likely nominee, GOP donors ended their work with the firm, and Democrats began paying Fusion GPS for opposition research on Trump.

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Fusion GPS commissioned Christopher Steele, a former British spy, to compile the dossier.

The firm's role in developing the controversial document has emerged as a topic of interest for the Senate Judiciary Committee, which is among several congressional panels probing Russia's efforts to meddle in the presidential election and possible ties between the Trump campaign and Moscow.

Simpson had initially declined to speak with the committee but agreed to do so last month. In exchange for his testimony, the panel's top lawmakers, Sens. Chuck Grassley Charles (Chuck) Ernest GrassleyGardner signals support for taking up Supreme Court nominee this year Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee McConnell digs in on vow to fill Ginsburg's Supreme Court seat MORE (R-Iowa) and Dianne Feinstein Dianne Emiel FeinsteinBiden leads Trump by 12 points among Catholic voters: poll Names to watch as Trump picks Ginsburg replacement on Supreme Court McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (D-Calif.), agreed to withdraw a subpoena for his testimony.

According to a Fox News report, Fusion GPS has already turned over some 40,000 documents to the judiciary committee.

Trump has repeatedly denied any collusion between his campaign and Russian officials and has fiercely denied allegations made in the dossier, which remains uncorroborated.