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.
It wasn't clear whether President Trump was basing his tweet on direct knowledge that he is under investigation, or on reports this week that special counsel Robert Mueller is examining whether the president obstructed justice by firing Mr. Comey last month amid the ongoing Russian Federation investigation.
Last week, former FBI Director James Comey told Congress he believed he was sacked by Trump to undermine the agency's Russian Federation investigation.
The US president's attack, which appears to target Rod Rosenstein, the deputy attorney general whois leading the government investigation, reflects Mr Trump's mounting frustration with the unrelenting controversy that has consumed his early presidency.
Reports have been mounting that Mueller is investigating Trump for potential obstruction of justice related to his firing of Comey.
As Bob Mueller, the special investigator now heading the FBI's probe into allegations of Russian election meddling, filled out his legal team and prepared to begin interviews, Trump erupted furiously for a second consecutive day.
Brand was confirmed as the No. 3 Justice Department official on a 52-46 vote in the Senate on May 18, with Democrats lining up against her.
Trump published a series of tweets in which he decried both the alleged investigation and media coverage thereof.
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Vice President Mike Pence has also hired a private lawyer to represent his interests in the expanding probe.
President Donald Trump, with National Security Adviser H.R. McMaster, waves as they head to Marine One.
Sekulow, American Center for Law and Justice's chief counsel, tried to downplay the content of the tweet and defended Trump's firing of Comey as a constitutional act.
WASHINGTON - President Donald Trump's Washington hotel saw nearly $20 million in revenue during its first few months of operation - a period that coincided with his election and inauguration as the 45th president.
Trump hadn't publicly admitted he was personally under investigation until now. Those associates include former national security adviser Michael Flynn, campaign chairman Paul Manafort, foreign policy aide Carter Page and outside adviser Roger Stone.
California Sen. Dianne Feinstein said she is concerned that Trump might try to fire both Rosenstein and Mueller.
Meanwhile, another lawyer, Washington veteran John Dowd, has joined the Trump legal team. "The president issued that tweet, that social media statement, based on a fake report, a report with no documented sources, from The Washington Post".