Trump, however, referred to the Post's reporting as a "phony story" in a tweet Thursday morning.
Although he was strongly critical of some of Comey's testimony, the president said last week that the former Federal Bureau of Investigation chief had vindicated him when he said that while he was at the agency, Trump was not the subject of the FBI's Russian Federation probe.
Comey said it should be special counsel Robert Mueller's task to determine whether Trump had obstructed justice, and therefore, had opened himself up to possible impeachment proceedings, keeping the allegations on Russian Federation in mind.
Trump lashed out on Twitter early Friday at a top Justice Department official overseeing the inquiry, reflecting his mounting frustration.
This is the first time the President is taking aim at Rosenstein, a Trump nominee who previously served as the U.S. Attorney for the state of Maryland and who was widely seen as being non-partisan.
The tweet also served as the first on-record confirmation by Trump that he is under investigation for obstruction of justice for firing Comey.
Trump did not respond to shouted questions from the media as he was leaving the White House lawns to board Marine One for the Joint Andres Air Force Base for his day-long trip to Florida.
The top Democrat on the Senate Judiciary Committee says she is "increasingly concerned" that President Donald Trump will attempt to fire Special Counsel Robert Mueller and Deputy Attorney General Rod Rosenstein. Nor did he deny he has entered the miniscule ranks of sitting presidents who have become the subject of a criminal investigation.
Vice President Mike Pence has also hired a private lawyer to represent his interests in the expanding probe.
Comey told Congress last week that he leaked his memos of his conversations Trump to a friend after a tweet by the president suggested he may have taped the conversations.
Friday's tweets are the latest in a week of angry social media responses by the president over a report by The Washington Post that Mueller was looking into whether Trump obstructed justice. She said it was inappropriate to fire Comey given that he was overseeing the Russian Federation investigation.
58 people missing and presumed dead in London fire; number may increase
There were also indications that political groups had joined protests that spread beyond those immediately affected by the fire. Police Commander Stuart Cundy said Saturday that this number, which was based on reports from the public, may rise.
"You are witnessing the single greatest WITCH HUNT in American political history - led by some very bad and conflicted people!" the president tweeted.
The White House has denied any collusion, and Trump has repeatedly complained about the probe, saying Democrats can not accept his election win. It wasn't clear whether he was basing his tweet on direct knowledge or on media reports.
Comey also testified he had told Trump he was not under investigation. Regulations way down, jobs and enthusiasm way up!
He followed 20 minutes later with a post touting his own "very powerful Social Media" that lets him bypass "The Fake News Media".
"Well, we were asked our opinion, and when we expressed it, which was consistent with the memorandum and the letter we wrote, I felt comfortable. and I guess the deputy attorney general did too", he said, "in providing that information in writing".
Trump is an avid user of Twitter, with over 32 million followers on his personal account and more than 18 million people on the official presidential account.
In another tweet earlier Friday, Trump writes, "After 7 months of investigations & committee hearings about my 'collusion with the Russians, ' nobody has been able to show any proof".
Democrats and Republicans alike have criticised his use of Twitter, particularly amid the investigation into Russia's interference in last year's presidential election.
Yet, the president's tweet contradicts his own comments about how and when he made a decision to fire Comey. Mueller is now leading the investigation into whether Russian Federation interfered in the 2016 presidential election.
Trump has called The Washington Post report a "phony story" and a "WITCH HUNT". He has questioned why investigators don't dig into the links between the Democrats and the Russian government, including his general election opponent, Hillary Clinton.
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