President Trump on Thursday criticized reports that he is now under investigation for obstruction of justice in connection with the firing of FBI Director James Comey.

"They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story," Trump tweeted. "Nice."

@realDonaldTrump They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice

@realDonaldTrump You are witnessing the single greatest WITCH HUNT in American political history - led by some very bad and conflicted people! #MAGA

Trump appeared to be a responding to a Washington Post report that special prosecutor Robert Mueller "is interviewing senior intelligence officials as part of a widening probe that now includes an examination of whether President Trump attempted to obstruct justice," officials said.

While the Justice Department appointed Mueller to investigate Russian efforts to interfere in the 2016 presidential election — and any "collusion" with the Trump campaign — his assignment also includes "any matters that arose or may arise" from the Russia probe.

Read more from USA Today:

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Comey told the Senate Intelligence Committee that he believes Trump fired him last month because he resisted a request to drop the Russia investigation, especially as it related to former national security adviser Michael Flynn.

Trump said he fired Comey for poor performance, though he also reportedly told a group of Russian officials that the Comey dismissal would help get the Russia probe behind him. He also called Comey a "nut job."

Deputy Attorney General Rod Rosenstein appointed Mueller as special counsel shortly after Comey's firing. Rosenstein was acting, because Attorney General Jeff Sessions had recused himself from acting in the Russia investigation after it was revealed he had held two meetings with Russian Ambassador Sergey Kislyak before the Nov. 8, 2016 election.

News that Mueller is now investigating Trump for obstruction of justice comes amid reports that Trump has discussed whether to remove the special counsel. White House officials said the president is not considering such a step.

Supporters of Trump condemned the news leaks regarding the new investigation.

"The FBI leak of information regarding the President is outrageous, inexcusable and illegal," said Mark Corallo, a spokesman for Trump's legal team.

It is possible, however, that the sources for the Post report could have been those interviewed as part of the investigation, not the investigators themselves.