For the past year, the Justice Department’s inspector general, Michael Horowitz, has been examining the FBI’s efforts to surveil a one-time Trump campaign adviser based in part on information from Christopher Steele, a former British MI6 agent who had worked with the bureau as a confidential source since 2010.

Several people interviewed by the Inspector General’s office over the past year tell POLITICO that Horowitz’s team has been intensely focused on gauging Steele’s credibility as a source for the bureau. One former U.S. official left the interview with the impression that the Inspector General’s final report “is going to try and deeply undermine” Steele, who spent over two decades working Russia for MI6 before leaving to launch his own corporate intelligence firm.

Thursday’s planned release of the full Mueller report by the Justice Department could shed new light on Steele’s role, and the veracity of the controversial dossier he assembled in 2016, featuring explosive—and in some cases sexually lurid—charges of Kremlin influence over Trump and his associates.