Obstruction of justice is a potentially impeachable offense.
The Washington Post revealed this week that special counsel Robert Mueller has broadened the ongoing Russian Federation probe and is interviewing top officials about the circumstances surrounding Comey's firing.
Robert Mueller, a former FBI director, is leading an investigation into alleged Russian meddling in the 2016 USA presidential election and possible collusion with the Trump campaign. Days after that happened, Trump told NBC News' Lester Holt that the Russian Federation investigation played a role in his decision to terminate him, and he has dismissed the probe as "fake news".
Trump said that "nobody has been able to show any proof yet". After Mueller was appointed last month, Trump tweeted: "This is the single greatest witch hunt of a politician in American history". Trump administration officials often say, typically while pointing to the performance of stock markets, businesses feel more confident under Trump.
As special counsel Robert S. Mueller III widens his inquiry of Russia's role in the 2016 presidential campaign, it can be hard to keep track of who is under investigation for what.
It was unclear whether his tweet about being under investigation was based on direct knowledge or new media reports that suggest Mueller is examining whether the president obstructed justice by firing Comey.
He reiterated tweets from Thursday morning about the investigation.
The president complained the probe is unfair and wondered why his defeated Democratic opponent was not getting the same scrutiny.
The Twitter attacks came as Vice-President Mike Pence hired a personal lawyer to represent him in the intensifying investigation.
Richard Cullen is a former Virginia attorney general and a former USA attorney for the eastern district of Virginia.
Mueller's investigation appeared to be reaching a broadening circle of current and former officials.
The Washington Post reported on Wednesday that investigators were looking into possible obstruction of justice. He is reportedly interviewing three top intelligence officials - Director of National Intelligence Dan Coats, National Security Agency Director Mike Rogers and retired deputy NSA director Richard Ledgett - as part of the probe as early as this week.
James B. Comey, who led the law enforcement investigation until he was sacked as FBI director May 9, testified last week before the Senate Intelligence Committee that he has no doubt that Russian Federation attempted to influence the presidential race by hacking the Democratic National Committee and launching cyberattacks on state election systems, among other tactics. He said he believes Mr Trump ultimately fired him "because of the Russian Federation investigation".
Mueller met Wednesday with the leaders of the Senate Intelligence Committee in an effort to ensure their investigations don't conflict.
The letter said the investigation will also probe Comey's testimony that Loretta Lynch, as President Barack Obama's attorney general, had directed him to describe an FBI probe into Hillary Clinton's email practices as merely a "matter" and to avoid calling it an investigation.