The New York Times sued the FBI on Friday, seeking a court's help in getting the agency to turn over documents created by fired FBI Director James Comey after his conversations with President Donald Trump.
In a series of statements on Twitter on Thursday, Mr Trump repeated his claim that Mr Mueller's investigation is a "WITCH HUNT" based on the "phony" premise of possible collusion between Russian Federation and a cadre of Trump campaign associates.
"He is a man of extraordinary independence and integrity. and great character", Vice President Mike Pence told reporters on Wednesday, noting that the longtime Justice Department official "came to work, sat down and made his recommendation" to fire Comey.
Mr Trump, who is due to give a speech in Miami on Friday where he is expected to announce a shift in United States policy towards Cuba, also boasted about his deployment of social media, despite warnings by legal experts that his tweets may be used against him in the ongoing investigation.
"After 7 months of investigations & committee hearings about my 'collusion with the Russians, ' nobody has been able to show any proof".
That appeared to be the first public confirmation that Trump is being investigated for obstruction of justice - and a digital salvo trained at Deputy Attorney General Rod Rosenstein, who wrote a memo criticizing Comey.
The White House confirmed that Mueller was among the candidates Trump interviewed to replace Comey. Rosenstein last month appointed former FBI Director Robert Mueller as special counsel overseeing the probe.
Trump has reportedly been fuming over the Justice Department's handling of the Russian Federation investigation.
President Donald Trump walks with National Security Adviser H.R. McMaster from the Oval Office to Marine One on the South Lawn of the White House in Washington, Friday, June 16, 2017, for a short trip to Andrews Air Force Base, Md., then onto Miami. With just about a week to go before India's Prime Minister Narendra Modi is scheduled arrive in Washington to meet Trump on June 26, the U.S. capital is awash with intrigue and suspense, with frequent parlor talk about if and when Trump will throw in the towel and the prospect of a "President Pence".
The investigation appears to be reaching a broadening circle of current and former officials.
"Americans should be skeptical about anonymous allegations", Rosenstein said.
A source on Trump's legal team claimed the president was referring to a Washington Post story "from the other night" rather than confirming he is being investigated. In a letter to Trump, Sessions called for a "fresh start" at the Federal Bureau of Investigation and recommended Comey's removal. Reports have alleged that Trump asked both men to publicly quash the credibility of the Russian Federation investigation.
Meanwhile, the top attorney for the Trump transition team has ordered the organization's staff to preserve all records related to Mueller's widening investigation.
Through a spokesman, Trump's personal attorney, Marc Kasowitz, blasted the Post's report, though he stopped short of denying it.