James Comey had concerns about Deputy Attorney General Rod Rosenstein before he wrote a memo which the Trump administration used to fire him as FBI director, according to a report Thursday.

Benjamin Wittes, a friend of Comey’s, told the New York Times that he and Comey had a prior discussion on the prospect of the Senate confirming Rosenstein to serve as deputy attorney general. Before joining the Justice Department, Rosenstein served as U.S. attorney for the District of Maryland.