"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 be questioning the integrity of Deputy Attorney General Rod Rosenstein, the U.S. Justice Department's No. 2 official. Rosenstein appointed Robert Mueller on May 17 as special counsel to head the inquiry into the Russia matter.
Rosenstein wrote a memo in May to Trump strongly criticizing FBI Director James Comey's performance. 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. Trump said last week he felt vindicated by Comey's June 8 testimony that he was not the subject of investigation while Comey headed the agency. Several congressional committees are also investigating the Russia matter.
ABC News reported on Friday that Rosenstein has said privately he may need to recuse himself from matters relating to the Russia probe because he could become a 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.
Rosenstein currently has such authority because Attorney General Jeff Sessions recused himself on March 2 after revelations of previously undisclosed meetings with Russia's ambassador to Washington while he was a Trump campaign advisor.
The Democratic National Committee said it saw a need for Rosenstein to recuse himself, but it said control over the investigation should be given to Mueller and not another Trump appointee.
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 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.
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.
The memo to former transition team members on Thursday also seeks specific information on five people, the Times reported. They are: Flynn; Paul Manafort, Trump's former campaign manager; Rick Gates, Manafort's business partner; Carter Page, a former foreign policy adviser; and Roger J. Stone Jr., an informal adviser to Mr. Trump.
Flynn was dismissed by Trump in February after it emerged he had misled Pence about conversations with the Russian ambassador. Manafort, Page and Stone have also been linked to the Russia investigations.
During an appearance in Florida on Friday, Vice President Mike Pence said his hiring of his own lawyer to help him in the Russia inquiry was a "very routine" move. Pence has hired Richard Cullen, a lawyer known for defending government officials in high-profile investigations, his office said on Thursday.
(Reporting by Susan Heavey, Lawrence Hurley, David Alexander, Dustin Volz, Roberta Rampton and Julia Edwards Ainsley in Washington; Writing by Will Dunham; Editing by Frances Kerry)
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)