"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt," Trump wrote on Twitter.
Trump did not specify who he was referring to, but he appeared to mean Deputy Attorney General Rod Rosenstein, the No. 2 official at the U.S. Justice Department. Rosenstein appointed Robert Mueller on May 17 as special counsel to head the inquiry into the Russia matter.
Rosenstein was also the author of a letter in May to Trump criticizing the performance of FBI Director James Comey. While the Trump administration initially said that letter was the reason the president fired Comey on May 9, Trump later said he did so because of the "Russia thing."
Comey told a Senate panel last week he believed Trump fired him because of the Federal Bureau of Investigation's Russia probe. Comey also testified Trump had directed him to drop a related FBI investigation into former national security adviser Michael Flynn.
A person familiar with Mueller's inquiry, speaking on condition of anonymity, said on Thursday Mueller was looking into whether Trump or others attempted to obstruct the Russia investigation.
The Republican president has repeatedly complained about the probe, calling it a witch hunt and saying Democrats cannot accept his election victory. But he had said last week he felt vindicated by Comey's testimony on June 8 that he was not the subject of investigation while Comey was heading the agency. Several congressional committees are also investigating the Russia matter.
In another development, members of Trump's transition team that served him after he was elected in November until he took office in January have been ordered to preserve materials related to the Russia matter, the New York Times reported.
Citing a memo from the general counsel's office of Trump's transition team, the Times said members were given the order on Thursday for any information involving Russia or Ukraine in the latest sign of the investigation's expanding reach.
ABC News reported on Friday that Rosenstein has privately said he may need to recuse himself from matters relating to the Russia probe, given that he could become a potential witness in the investigation.
Citing unnamed sources, ABC reported Rosenstein told Associate Attorney General Rachel Brand she would have authority over the probe if he were to step aside.
A Trump confidant said this week the president had considered firing Mueller. Rosenstein, the official who would be responsible for dismissing Mueller, told U.S. lawmakers he would fire him only with good cause.
U.S. intelligence agencies concluded in a report issued in January that Russia interfered in the presidential race to try to help Trump win, in part by hacking and releasing emails damaging to Democratic candidate Hillary Clinton.
Moscow has denied any interference. The White House has denied any collusion.
NO PROOF, SAYS TRUMP
Trump, who hired his own lawyer last month to represent him regarding probes by the special counsel and congressional committees, kept up his criticism of the investigations in a series of tweets on Friday.
"After 7 months of investigations & committee hearings about my 'collusion with the Russians,' nobody has been able to show any proof. Sad!" he wrote in one post.
Vice President Mike Pence's office said on Thursday he had hired a lawyer known for defending government officials in high-profile investigations to help him with the Russia probes.
Pence hired Richard Cullen, chairman of law firm McGuireWoods and a former federal prosecutor who has long ties to Comey, to help him respond to inquiries from Mueller, a spokesman said.
The memo to former transition team members on Thursday also seeks specific information on five people, the Times reported. They are Paul Manafort, Trump's former campaign manager; Rick Gates, Manafort's business partner; Carter Page, a former foreign policy adviser; Flynn; and Roger J. Stone Jr., an informal adviser to Mr. Trump.
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. Manafort, Page and Stone have also been linked to the Russia investigations.
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