A key figure behind the so-called dossier featuring uncorroborated and salacious allegations about then-candidate Donald Trump’s ties to Russia will be questioned by investigators from the Senate Judiciary Committee today about the funding and sources for the document.

During last year’s heated Republican primary race, Fusion GPS, a private research firm founded by former Wall Street Journal reporter Glenn Simpson, was initially paid about a million dollars by wealthy Republicans and then later worked for Democrats, all of whom wanted to dig up dirt on Trump and plant negative news stories, according to political operatives.

Simpson, who will appear in a closed session on Capitol Hill on Tuesday, hired the former MI6 agent Christopher Steele to compile the now infamous dossier, which alleged that members of the Trump campaign had colluded with Russian agents to damage Hillary Clinton, Trump’s Democratic opponent.