Allegations Kushner's finances are being studied as part of the investigation come after it was reported last month the team behind the probe were examining meetings he had with Russian Ambassador Sergey Kislyak and a Russian banking executive late previous year during the presidential transition.
Trump did not identify "the man" but appeared to be questioning the integrity of Deputy Attorney General Rod Rosenstein, the Justice Department's No. 2 official who appointed Mueller on May 17, supervises the probe and wrote a memo to Trump critical of Comey that preceded Comey's firing. But Mueller answers to Rosenstein, who did, in fact, write a memo in early May recommending that Trump fire FBI Director James Comey.
Mueller took over the DOJ Russia probe after a chain of events set in motion by Trump, Rosenstein and Attorney General Jeff Sessions. Rosenstein last month appointed former FBI Director Robert Mueller as special counsel overseeing the probe.
President Trump has continued to blast the investigation into Russian meddling as a "witch hunt" that has so far failed to produce any evidence that he colluded with Russians.
At a Senate Intelligence Committee hearing last week, Coats and Rogers refused to say whether Trump asked them to intervene in the investigation, which is also examining whether the Trump campaign team colluded with Russian Federation during last year's presidential campaign.
About a week later, Mueller, another former Federal Bureau of Investigation director, was appointed, over Trump's opposition, as special counsel to lead the criminal probe.
Democrats and Republicans alike decried President Trump's tweet about the Russian Federation investigation on Capitol Hill Friday. "The Department of Justice has a long-established policy to neither confirm nor deny such allegations", Rosenstein's full statement read.
A person familiar with Trump's tweet said the President was referencing news reports that he is under investigation and was not indicating that he's been personally informed by the special counsel that he is a target of the probe.
Mr Trump, who is due to give a speech in Miami on Friday where he is expected to announce a shift in U.S. 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.
Trump says he's 'under investigation' over Comey firing
The Twitter attacks came as Vice-President Mike Pence hired a personal lawyer to represent him in the intensifying investigation. Since then Mueller, who served as FBI director before Comey, has taken over the investigation.
Mr Mueller was appointed as special counsel to investigate Moscow's involvement in the presidential election a year ago and possible collusion by Trump campaign associates with the Russians, but there has been no indication that Mr Mueller told the president to fire Mr Comey.
Comey was reportedly fired because despite Trump asking him to drop the Russian probe of Flynn, the FBI director still moved forward with the investigation.
Kushner and other Trump business associates are being looked at in the investigation, according to a new report.
The Twitter attacks came as Vice President Mike Pence hired a personal lawyer to represent him in the intensifying investigation. It wasn't clear whether he was basing his tweet on direct knowledge or on media reports.
But Republicans also worry that Mr. Trump is giving Mueller more material to investigate all the time.
Trump also retweeted law professor Alan Dershowitz, who has called for a halt to conversations about obstruction of justice because there was "no plausible case".
The White House confirmed that Mr. Mueller was among the candidates Mr. Trump interviewed to replace Mr. Comey. He also has millions of followers on his official Facebook pages.
"He now lives within himself, which is a risky place for Donald Trump to be", someone close to Trump said to the news network, "I see him emotionally withdrawing". Both Sessions and Rosenstein have refused to detail these interactions before Congress, citing privileged conversations.
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