It was the latest example of the president moving to publicly undermine an investigation he has criticized as politically motivated.
But Trump's confrontational style has also renewed fears that he may attempt to derail the inquiry by firing those leading it.
In one of a series of morning tweets, he complained he was “being investigated for firing the
Trump is probably pointing to Deputy Atty. Gen. Rod Rosenstein, who named
The Washington Post reported Wednesday that Mueller has expanded that investigation to include whether Trump attempted to obstruct the inquiry by firing former FBI director James. B. Comey.
Rosenstein told a Senate panel this week that he hadn't spoken to Mueller about the substance of the investigation since he appointed him, adding that Mueller should make his own determinations about what is within the scope of it.
Rosenstein appointed Mueller after Trump fired Comey, a decision that the White House initially said was largely based on a memo written by Rosenstein outlining concerns about Comey's leadership — but that never explicitly recommended his termination, despite what the president's tweet Friday suggests.
Trump told NBC News in an interview last month that he was planning to fire Comey anyway, independent of Rosenstein's memo.
White House officials would not comment on whether Trump was confirming a possible obstruction of justice case against him or simply commenting on the reports of one, referring all questions to his personal attorney. But it was another example of him commenting about the larger investigation in a way that frustrates his personal attorneys and administration officials.
It also comes amid speculation that Trump is considering firing Mueller — a course of action congressional Republicans are widely advising against, though some Trump allies, like former House Speaker Newt Gingrich, support.
After initially declining to deny the report, the next day Deputy White House Press Secretary Sarah Huckabee Sanders said, "While the president has the right to, he has no intention to do so."
Trump's tweet Friday apparently referring to Rosenstein, who was appointed by Trump and confirmed by the Republican-led Senate overwhelmingly earlier this year, comes after the deputy attorney general issued a cryptic statement Thursday night advising Americans to "exercise caution before accepting as true any stories attributed to anonymous 'officials.'"
That statement appeared to be linked to a separate report that Mueller is also investigating the business dealings of the president's son-in-law, senior advisor Jared Kushner.
Also Friday, the Justice Department said Rosenstein saw no reason now to recuse himself from supervising Mueller.
"As the deputy attorney general has said numerous times, if there comes a time when he needs to recuse, he will," said Ian Prior, a department spokesman. "However, nothing has changed."
The statement came after ABC News reported that Rosenstein had discussed the possibility that he might have to step aside from overseeing the special counsel's office because of his role in Comey's firing.
Trump’s latest tweets have Sen.
"The message the president is sending through his tweets is that he believes the rule of law doesn't apply to him and that anyone who thinks otherwise will be fired. That's undemocratic on its face and a blatant violation of the president's oath of office," Feinstein said in a statement Friday.
"It's becoming clear to me that the president has embarked on an effort to undermine anyone with the ability to bring any misdeeds to light, be that Congress, the media or the Justice Department. The Senate should not let that happen. We're a nation of laws that apply equally to everyone, a lesson the president would be wise to learn," she said.
Feinstein stressed that only Rosenstein has the authority to fire Mueller, not the president.
"If the president thinks he can fire Deputy Atty. Gen. Rosenstein and replace him with someone who will shut down the investigation, he's in for a rude awakening. Even his staunchest supporters will balk at such a blatant effort to subvert the law," Feinstein said.
Meanwhile, the inquiry continues to pose legal challenges to those in the administration.
A spokesman for Vice President
Richard Cullen, a former federal prosecutor and currently chairman of the McGuireWoods law firm, will help Pence handle inquiries from Mueller, said Jarrod Agen, Pence's communications director, in a statement.
Pence, who joined Trump in Miami on Friday, told a reporter that the move was "very routine."
In recent days, Mueller reportedly has asked to talk to senior Trump administration officials as the team of investigators looks into whether people working on Trump's presidential campaign colluded with Russian officials to interfere in the election process.
"The vice president is focused entirely on his duties and promoting the president's agenda and looks forward to a swift conclusion of this matter," Agen said.
In late May, Trump hired Marc Kasowitz to handle legal and media requests about the Russia investigation.
Michael Cohen, one of Trump's personal attorneys, has hired Stephen M. Ryan, a Washington-based lawyer from McDermott, Will & Emery, who has experience prosecuting criminal cases as an assistant U.S. attorney, according to NBC News.
Times staff writers Brian Bennett and Joseph Tanfani contributed to this report.
For more White House coverage, follow @mikememoli on Twitter.