Comey notified Donald Trump in the fall of 2016 that the FBI had received "materials" alleging deviant behavior and financial misconduct tied to Russia. 

The "Steele Dossier," created by former MI6 spy Christopher Steele on behalf of Clinton-commissioned opposition research firm Fusion GPS, was notably used as the basis for a Foreign Intelligence Surveillance Court (FISA) warrant to spy on Trump campaign associate Carter Page (and presumably, via unmasking, anyone he communicated with) as part of a sweeping, ongoing counterintelligence operation which began during the 2016 election.