Comey was sacked on 9 May, and initially the White House cited his handling of the investigation into Hillary Clinton's private email server as the reason for his termination, releasing a memo authored by deputy attorney general Rod Rosenstein as a basis for this action.
The White House has said the president "has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn".
Attorney General Jeff Sessions is also under investigation as a member of the Trump campaign, leading Sessions to recuse himself from the case and allowing Rosenstein to appoint Mueller to head the investigation, reports Fox News. As CNN Politics reporter Chris Cillizza noted, it will be much harder for Trump and his colleagues to dismiss the FBI's probe into Russia's involvement as unnecessary meddling.
Trump has regularly turned to Twitter to blast reports about his alleged connections to Russia or the inquiries into possible collusion between the Trump campaign and Russian operatives.More news: American Brooks Koepka wins US Open
Trump has furiously railed against leaks coming out of his administration and the intelligence community since he took office. "Sad!" Trump said in his first tweet of the day.
Trump also took aim at his favorite foil - the media.
"Why is that Hillary Clintons family and Dems dealings with Russian Federation are not looked at, but my non-dealings are?", Trump tweeted just before 4 p.m. EDT.
He had likened the appointment of the special counsel with the "single greatest witch hunt of a politician in American history".
In this last tweet, Trump was referring to a memo written by Rosenstein that was intensely critical of Comey and implicitly recommended his removal.