In a statement at the time, the White House, on the President's behalf, said, "As I have stated many times, a thorough investigation will confirm what we already know - there was no collusion between my campaign and any foreign entity".
He also has the unquestioned authority to call any and all members of Trump's staff to ask them about the administration's inner workings and to produce all relevant documents. The report did not name the adviser.
Trump has denied the conversation took place.
Because it was a closed briefing, it's unclear exactly how Rosenstein described how he knew Trump would fire Comey.
Deputy U.S. Attorney General nominee Rod Rosenstein waits to testify before the Senate Judiciary Committee March 7, 2017 in Washington, DC.
On Thursday, after a briefing with Rosenstein on Capitol Hill, Sen.
Rosenstein's appointment gives Mueller the authority to probe into Russian Federation and other "related matters".
Democratic Rep. Seth Moulton of MA echoed that sentiment, saying there was "considerable frustration" and that "he refused to answer a lot of questions".
"There needs to be a relationship with that investigation where they can say with confidence, 'This really is a problem for us, ' or 'Hey, we'd rather you didn't do it, but it's not a problem, ' " Blunt said.More news: Trump willing to try engagement with North Korea, on conditions: Seoul
Still, he said he believed that Comey "made his decisions in good faith". Trump has insisted at times that the decision was his alone, but he also has pointed - as recently as Thursday - to the "very strong" recommendation from Rosenstein.
"I'm fine with whatever people want to do", he added.
"I haven't seen any evidence of actual collusion".
Mr Rosenstein, in separate addresses to the House and Senate this week, said that, in spite of a personal admiration he felt for Mr Comey, he believed the then-FBI director had breached protocol and politicised an investigation into former Secretary of State Hillary Clinton's use of a private email server while in government. Rosenstein denounced that decision as "profoundly wrong and unfair".
Following the appointment of Robert Mueller as special counsel to lead the investigation into possible connections between the Trump campaign and Russian Federation, few people have the perspective on the process about to unfold like Peter Zeidenberg.
Trump has reacted furiously to the appointment but Democrats and even many Republicans have generally welcomed it. However, at a combative news conference Thursday, he fell short in trying to resolve questions about investigations into his campaign and his first four months in office. "In the meantime, I will never stop fighting for the people and the issues that matter most to the future of our country", he had said.
Democratic senators had been prepared to press Rosenstein to take the step of appointing a special prosecutor, but were left praising him instead before his closed-door briefing began.
At his March confirmation hearing for the job of deputy attorney general, Rosenstein refused to commit to the appointment of a special counsel to oversee an investigation into Russian interference in the 2016 presidential election, saying he was aware of no requirement to recuse himself and had no first-hand knowledge of the probe anyway.
Republican representatives have grown increasingly anxious since Mr Trump sacked Mr Comey, who had been leading the bureau's probe.
Democrats who had hoped to used the deputy AG's discontent over being fingered as the primary reason for Comey's firing are, to say the least, a little frustrated at the moment.