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.

Wittes said he approved of Rosenstein, but to his surprise Comey had reservations.

"He said, 'I don't know. I have some concerns. He's good, he's solid but he's also a survivor and you don't survive that long without making some compromises and I'm concerned about that,'" Wittes recounted to the New York Times.

Comey was fired by Trump last Tuesday, weeks after this conversation reportedly took place, amid conflicting accounts about how the decision was reached.

Trump claims that he was "going to fire" Comey no matter what memos from his Justice Department said. Democrats have pointed to that as evidence that Trump was trying to hinder the FBI's probe into Russia, but Trump and other Republicans say they were unhappy with Comey's failure to charge Hillary Clinton for her use of private emails and the resulting leak of classified information.