Late Friday, the Senate intelligence committee announced that Comey had agreed to testify at an open hearing at an undetermined date after Memorial Day.
Former FBI chief Robert Mueller was tasked on Wednesday with leading a beefed-up investigation into whether Mr Donald Trump's team colluded with Russian Federation to tilt the 2016 election in the president's favour.
But at the very least, it's getting tougher and tougher for Trump to make the claim that this is a complete nothing-burger, conjured from nothing more than Democrats' hard feelings about losing the 2016 election - which Trump argued Thursday.
Grant Woods, who served as a special prosecutor in several Arizona cases after serving as the state's attorney general, agreed.
Deputy Attorney General Rod Rosenstein learned the day before President Donald Trump fired FBI Director James Comey that the law enforcement chief was going to be sacked. "We want to be careful we don't interfere with any criminal investigations", McCaskill said. "He'll do it and he'll do it professionally".
The committee is one of several on Capitol Hill investigating possible collusion between Russian Federation and President Donald Trump's 2016 campaign.
Senate Minority Whip Dick Durbin, D-Illinois, echoed the revelation about Comey's firing when he emerged from the briefing, saying that while Rosenstein wouldn't go into detail, he said the president declared on May 8 "that he was going to fire Comey...on May 9, [Rosenstein] wrote his memo". "This is the single greatest witch hunt of a politician in American history!"
Mr Trump said he was about to name a replacement for Mr Comey, another effort to settle the waters, with former senator Joe Lieberman seen as the front-runner.
Wednesday night, The New York Times reported that Flynn warned the Trump administration weeks before he started his White House job that he was under FBI investigation for work he had done on behalf of Turkey's government during the campaign.
But Thursday morning's tweets indicate that Trump will handle the news of a special counsel the way Trump handles all controversies - with a vigorous and potentially self-destructive fight.
House investigators said they were plowing full speed ahead with their own inquiry - one source noted that the appointment of a special counsel didn't matter much because Comey had not been providing House investigators much access to begin with. That would surely include Trump's firing last week of FBI Director James Comey.
"It is hard to see a friend of yours being in the middle of that", said Jan Paul Miller, a St. Louis attorney who worked with Rosenstein when both were assistant USA attorneys in Maryland.
"You're pretty well knocked out of the game", he said about the ongoing high-profile congressional probes, "and that's probably the way it should be". The White House counsel informed the President, reports CNN.
Asked point-blank if he'd done anything that might merit prosecution or even impeachment, Trump said no - and then added of the lingering allegations and questions: "I think it's totally ridiculous". Trump aides initially pointed to a memo written by Rosenstein that had been critical of a number of Comey actions prior to Trump coming into office.
Lindsey Graham said he believes the investigation has evolved into a criminal investigation.
Republican Sen. Richard Burr of North Carolina says ousted National Security Adviser Flynn's attorneys "have not yet indicated their intentions regarding the Senate Intelligence Committee's subpoena" as part of the panel's probe into Russia's meddling in the 2016 presidential election.
Asked if he urged Mr Comey at a February meeting to drop his probe of the Russian Federation connections of Mr Trump's national security adviser, Michael Flynn, the president said: "No. No". "There is no collusion between myself and my campaign".
"I applaud the Trump Administration for taking this important step that the previous Administration repeatedly declined to do in other matters, ensuring an independent and thorough investigation is conducted to unearth all the facts".