US President Donald Trump told Russian officials at a meeting last week that James Comey was a "nut job" whose removal as FBI director relieved "great pressure" on him, according to The New York Times.
The Times cited notes from a May 10 Oval Office meeting, the day after Mr Trump fired Mr Comey.
Separately, The Washington Post reported that the FBI investigation into possible co-ordination between Russia and the Trump presidential campaign was moving closer to the White House.
Law enforcement officials now consider a senior Trump adviser a "person of interest" in the probe, the Post reported.
The developments were a blow to White House efforts to dampen down interest in the Russia investigation as Mr Trump and his staff boarded Air Force One for Saudi Arabia, the first stop on his first foreign trip as president.
The details of his comments to the Russians would seem to bolster theories that Mr Trump fired Mr Comey in an effort to choke off the Russia investigation.
Earlier this week, the justice department appointed former FBI director Robert Mueller to take over the federal investigation in an effort to re-establish independence from the White House.
Deputy attorney general Rod Rosenstein told Congress on Friday he stands by a memo he wrote bluntly criticising Mr Comey.
But he made clear it was not his intention for Mr Trump or other White House officials to use the document to justify firing Mr Comey, which is what they have done.
In closed-door meetings with politicians on Thursday and Friday, Mr Rosenstein said he wrote the memo after Mr Trump told him one day before the May 9 firing that he wanted to dismiss Mr Comey.
Mr Rosenstein said that though he was personally fond of Mr Comey, "I thought it was appropriate to seek a new leader".
The justice department on Friday issued the text of Mr Rosenstein's opening remarks for the briefings on Capitol Hill.
That was two days after Mr Rosenstein named Mr Mueller as a special counsel to investigate possible co-ordination between Russia and the Trump campaign to influence the 2016 presidential election.
The White House has struggled since Mr Comey's firing to explain the chain of events that led to it and who exactly made the decision.
Mr Trump has insisted at times that the decision was his alone, but he has also pointed - as recently as Thursday - to the "very strong" recommendation from Mr Rosenstein.
Mr Rosenstein made it abundantly clear to the politicians that he drafted his memo only after Mr Trump told him of his plans to dismiss the FBI director.
"My memorandum is not a statement of reasons to justify a for-cause termination," he said.
But he added: "I wrote it. I believe it. I stand by it."
The memo focuses on Mr Comey's handling of the Hillary Clinton email investigation, particularly the FBI director's decision to divulge details to the public at various junctures.
Mr Rosenstein denounced that as "profoundly wrong and unfair".
House members and senators said Mr Rosenstein in his briefings steered clear of specifics in answering questions about his appointment of Mr Mueller but made clear the former FBI director will have wide latitude to pursue the investigation, potentially including criminal charges.
Mr Trump has reacted furiously to the appointment.
However, at a combative news conference on Thursday, he fell short in trying to resolve questions about investigations into his campaign and his first four months in office.
Asked point-blank if he had done anything that might merit prosecution or even impeachment, Mr Trump said no - and then added of the lingering allegations and questions: "I think it's totally ridiculous. Everybody thinks so."
The appointment of the special counsel indicates others believe that is still open to question.