The head of the firm behind the infamous anti-Trump dossier was interviewed for hours behind closed doors Tuesday on Capitol Hill, sources confirmed to Fox News. 

Glenn Simpson, the co-founder of investigative firm Fusion GPS, met with staff of the Senate Judiciary Committee as part of the committee’s investigation into Russian meddling and potential collusion with the Trump campaign in the 2016 presidential election.

A committee spokesperson did not give Fox News any further information on the meeting with Simpson and Judiciary staff, though a source confirmed that Fusion GPS has given the Senate committee over 40,000 documents.