“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,” Trump wrote on Twitter.

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

— Donald J. Trump (@realDonaldTrump) June 15, 2017

The Washington Post, citing unidentified officials, on Wednesday reported that special counsel Robert Mueller is investigating Trump for possible obstruction of justice.

Mueller is leading the Russian probe after being appointed by Deputy Attorney General Rod Rosenstein in the Department of Justice, which oversees the Federal Bureau of Investigation. Last week, former FBI Director James Comey told Congress he believes he was fired by Trump to undermine the agency’s Russia probe.

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

— Donald J. Trump (@realDonaldTrump) June 15, 2017

The obstruction of justice investigation into Trump began days after Comey was fired on May 9, according to people familiar with the matter, the Washington Post said.

Dan Coats, the director of national intelligence, Mike Rogers, the head of the National Security Agency, and Richard Ledgett, the former deputy director at the NSA, had agreed to be interviewed by Mueller’s investigators as early as this week, the Post said. It cited five people briefed on the requests who spoke on condition of anonymity.

Mark Corallo, a spokesman for Trump’s legal team, on Wednesday denounced the report, saying: “The FBI leak of information regarding the President is outrageous, inexcusable and illegal.”

A spokesman for Mueller’s team declined to comment on Wednesday.