The Washington Post reported this week that Mueller is investigating whether Trump attempted to obstruct justice when he suggested Comey drop an FBI investigation into Michael Flynn, Trump's former national security adviser, as well as when he fired Comey.
It is not clear, however, if Trump was referring in Friday's tweet to Mueller or Deputy Attorney General Rod Rosenstein, who wrote a letter to Attorney General Jeff Sessions recommending that the president fire Comey.
Trump's digital volley appeared to be trained at Deputy Attorney General Rod Rosenstein, who recommended the firing last month of James Comey - an nearly unprecedented move that could yet imperil Trump's presidency. A day later, Rosenstein appointed Mueller, who was at the helm of the Federal Bureau of Investigation during the September 11 attacks, as special counsel. Trump confirmed on his Twitter account that an investigation is being conducted against him for firing former FBI Director James Comey, most likely under the suspicion that this was an obstruction of justice.
Flynn, dismissed by Trump in February after it emerged he had misled Pence about conversations with the Russian ambassador, is a subject in investigations by intelligence committees in the House of Representatives and Senate, as well as the FBI.
On Thursday, it was reported that President Trump himself was under investigation for possible obstruction of justice. Trump tweets. Echoing tweets from Thursday, he concludes by adding: "Witch Hunt".
Senior Cabinet minister offer support to UK prime minister
Gove, a long-time opponent who was dismissed when May became prime minister a year ago , will now serve as environment secretary. Foreign minister Boris Johnson, who was reported by British media to be lining up a leadership bid, insisted May should stay.
As Bob Mueller, the special investigator now heading the FBI's probe into allegations of Russian election meddling, filled out his legal team and prepared to begin interviews, Trump erupted furiously for a second consecutive day. Since then Mueller, who served as FBI director before Comey, has taken over the investigation.
The tweet marked the first public, official confirmation that the president is under investigation for firing Comey in May. Pence's office confirmed he had retained Richard Cullen, a former Virginia attorney general and U.S. attorney for the Eastern District of Virginia, to assist "in responding to inquiries" from Mueller.
Mr Comey said he was "sure" Mr Mueller was looking at whether Mr Trump had obstructed justice, but added that to his knowledge, the president had not tried to impede the Russian Federation inquiry.
In a series of tweets, Trump lashed out against the investigation into whether his campaign colluded with Russian Federation to influence the 2016 election.