“I thought it was appropriate to seek a new leader,” he said, expressing more direct support for the firing than he had in his more measured memo, which stopped short of endorsing a particular action but rather outlined what he called Mr. Comey’s “serious mistakes” and noted that any possible decision to dismiss Mr. Comey “should not be taken lightly.”
Before submitting his memo, Mr. Rosenstein said, he solicited feedback from several career lawyers at the Justice Department, including one specializing in ethics issues. He told that lawyer Mr. Trump planned to fire Mr. Comey, and that he was a writing a memo addressed to Mr. Sessions “summarizing my own concerns.” The other lawyer was not identified.
The release of the memo followed what was otherwise an unremarkable appearance on Friday that left many House members frustrated by Mr. Rosenstein’s refusal to answer questions about the investigation into Russian election meddling.
Representative Seth Moulton, Democrat of Massachusetts, said there was “considerable frustration” among lawmakers as Mr. Rosenstein declined to answer more detailed questions about the events leading to the abrupt termination.
“This renewed my confidence that we should not have confidence in this administration,” Mr. Moulton said. Asked whether that included Mr. Rosenstein, he said, “I don’t think he did a lot to bolster our confidence in him.”
According to lawmakers and Mr. Rosenstein’s prepared remarks, the deputy attorney general offered little clarity about how related congressional inquiries may proceed in light of Mr. Rosenstein’s appointment on Wednesday of a special counsel, Robert S. Mueller III, to examine the possibility of collusion between President Trump’s associates and Russian officials.
The fact that Mr. Mueller’s inquiry is focused on possible crimes is almost certain to limit the cooperation of potential subjects of the investigation who might otherwise testify before Congress or share documents.
“Congress is going to want to look over the shoulder of this investigation,” said Representative Darrell Issa of California, a Republican who was an aggressive chairman of the House Oversight Committee during the Obama administration. “The executive branch will always try to limit that for fear it will contaminate potential criminal investigations, or leaks.”
“I don’t expect this to be any different,” he added.
Mr. Rosenstein instead delivered careful characterizations about the inquiry and deferred to Mr. Mueller’s autonomy as special counsel, and a pending investigation into Mr. Comey’s conduct by the Justice Department’s Inspector General. He said that his memo had not been a legal brief, or a finding of official misconduct by Mr. Comey or “a statement of reasons to justify a for-cause termination.”
Representative Jackie Speier of California, a Democrat on the House Intelligence Committee, said Friday that Mr. Rosenstein had made it clear that Mr. Mueller would have “carte blanche authority,” but he also reassured lawmakers that he understood the role of Congress.
“I think he’s very respectful of the role that the Congress is playing in doing its investigation,” she said, “and the separate and distinct role that the Department of Justice is pursuing.”
Mr. Rosenstein signed the legal order naming Mr. Mueller special counsel on Wednesday before informing Mr. Sessions or the White House of that decision. Democrats were simultaneously heartened by the selection of a respected former F.B.I. director and prosecutor to lead an investigation that they feared had been tarnished by Mr. Trump’s interference, while also concerned about the possibility of losing their grip on the information coming out of their own investigations.
The New York Times has reported that Mr. Trump asked Mr. Comey for his “loyalty” during a private dinner and requested he drop the investigation into Michael T. Flynn, his former national security adviser, who is under scrutiny regarding his ties to Russia and Turkey.