Rosenstein, according to his a copy of his prepared remarks at a briefing before the US House of Representatives, said that he learned on May 8 that Trump intended to fire Comey and that Trump sought Rosenstein's "advice and input" that same day.
The next day Rosenstein issued the memo to Trump, and Comey was swiftly dismissed.
While Rosenstein said he "chose the issues to include in my memorandum" and believed it was time for new leadership at the FBI, he also said his memo was "not a statement of reasons to justify a for-cause termination."
"I wrote it. I believe it. I stand by it," Rosenstein's statement said.
Members who attended Friday's briefing said Rosenstein was otherwise "very guarded" and "frustratingly cautious," deferring many answers to being under the purview of the recently appointed special counsel, Robert Mueller."It's clear he just wanted to defer to Mueller on everything tough," said one Democratic lawmaker inside the room.
Democratic Rep. Brad Sherman told reporters that it was a question Rosenstein faced more than 10 times. "He was explicitly non-responsive," Sherman said.
Democratic Rep. Seth Moulton of Massachusetts echoed that sentiment, saying there was "considerable frustration" and that "he refused to answer a lot of questions."
One new development from Friday's briefing: Rosenstein said news reports that Comey asked for and was denied more resources for the bureau's Russia probe were not true.
"Finally, I want to address the media claims that the FBI asked for additional resources for the investigation of Russian interference in the 2016 presidential election," Rosenstein said, according to the prepared remarks, which were released by the Justice Department. "I am not aware of any such request. Moreover, I consulted my staff and Acting FBI Director Andrew McCabe, and none of them recalls such a request."
Rosenstein's briefing also comes after the decision he made earlier this week to appoint Mueller to be special counsel in the Justice Department's probe of Russian meddling in the US election and any possible collusion between Moscow and Trump's presidential campaign last year.
Rep. Darrell Issa, R-California, said Rosenstein was asked about the timeline of Comey's firing and essentially replied, "Let the record speak for itself."
While it frustrated many Democrats, House Minority Whip Steny Hoyer said he understood why Rosenstein was guarded.
"I think Deputy Attorney General Rosenstein responded to the extent he thought appropriate within the context of the investigations that are ongoing," he said.
Rosenstein briefed senators Thursday afternoon in a classified, closed-door meeting. Senators coming out of that meeting told reporters that Rosenstein said Trump made it clear that he planned to fire Comey before
Rosenstein wrote a memo outlining issues with the former FBI director's flouting Justice Department protocol.
But Rosenstein did not disclose further details about the memo he wrote, according to House members on Friday. Democratic Rep. Rick Nolan of Minnesota said Rosenstein was asked repeatedly who told him to write it, and he would not say, adding that it was part of Mueller probe.
Democratic Rep. Jackie Speier of California said Rosenstein made clear Mueller has "carte blanche authority" in his investigation, and many members expressed confidence in Mueller, a former FBI director.
Rep. Elijah Cummings, the top Democrat on the House Oversight Committee, said Mueller was a "straight shooter" who will "bring some normalcy to this process."
"This is a fight for the soul of our democracy," he added. "We cannot afford to lose this one."
Majority Whip Steve Scalise told reporters that if the special counsel gets to a point where the investigation is interfering, then they will have that conversation. At this point, however, Scalise affirms that the House intelligence committee will continue doing what it's been doing.
"So far, there's been no suggestion that we cannot continue to go forward," Scalise told reporters.
Rep. Emanuel Cleaver told reporters he might be the only Democrat who feels better after the briefing and felt "higher level of comfort" after hearing that Rosenstein is giving Mueller wide latitude "including latitude to investigate him" on how he handled Comey's firing.
He said both Republicans and Democrats both applauded when he responded to a question about the ability of Mueller to decide the contours of the investigation. Cleaver said Rosenstein told House members"this is not a partisan investigation, this is about Russia involvement in our democracy."
"It was a powerful moment."