Last spring, acting FBI Director Andrew McCabe wrote a confidential memo regarding a conversation he had at the Justice Department with Deputy Attorney General Rod Rosenstein about President Trump's abrupt firing of McCabe's predecessor, James Comey. Rosenstein told McCabe that Trump had originally asked him to reference Russia in a memo he used to justify firing Comey, several people familiar with the discussion told The New York Times.
Rosenstein's memo instead took Comey to task for the way he handled the investigation into Hillary Clinton's emails. McCabe has reportedly given his memo and other documents to Special Counsel Robert Mueller, who is investigating whether Trump attempted to obstruct the probe into his campaign's ties to Russia. Rosenstein never told McCabe what exactly Trump wanted him to say about Russia, the Times reports, but McCabe felt the anecdote could be potential evidence that Comey's firing was related to the FBI's investigation into Trump and Russia, and that Rosenstein was providing cover for Trump by writing about the Clinton probe.
McCabe was fired in March right before his retirement, after an internal investigation determined he released information to the media that he shouldn't have. He says this was politically motivated, an attempt to discredit him as a witness in the Mueller investigation.