Sessions told Trump in a letter that he believes "a fresh start is needed at the leadership of the FBI."
In a memorandum titled "Restoring public confidence in the FBI," Rosenstein said he couldn't defend Comey's handling of the investigation into Hillary Clinton's emails.
"The Director was wrong to usurp the Attorney General's authority on July 5, 2016, and announce his conclusion that the case should be closed without prosecution. It is not the function of the Director to make such an announcement," the deputy attorney general said.
Last summer, Comey said "no charges are appropriate" in the FBI's investigation of Clinton.
"Although there is evidence of potential violations regarding the handling of classified information, our judgment is that no reasonable prosecutor would bring such a case," he said in July.
Rosenstein said that the dismissed FBI director compounded the error when he "ignored another longstanding principle: we do not hold press conferences to release derogatory information about the subject of a declined criminal investigation."
The deputy attorney general was referring to Comey's letter to Congress, which said the FBI was probing an additional batch of emails related to the Clinton investigation. Comey later announced that the FBI had "not changed its conclusions" after reviewing the new cache of emails.
The former Democratic nominee has cited Comey's letter, coming a little more than a week before the 2016 presidential election, as a contributing factor to her loss.
Rosenstein said that such information may be disclosed in legal proceedings, but that it's not something that should be done "gratuitously."
"The Director laid out his version of the facts for the news media as if it were a closing argument, but without a trial. It is a textbook example of what federal prosecutors and agents are taught not to do," he said.
Comey has defended his actions, saying that he was simply trying to not conceal the FBI's decision to investigate the additional emails.
But Rosenstein refuted Comey's language, saying that federal agents "are not concealing anything" when conducting an investigation quietly.
"In that context, silence is not concealment," the deputy attorney general said.
Here are Sessions' letter and Rosenstein's memo on Comey's removal: