Trump further claimed it all stems from his critics' "phony collusion" allegations. 

The Washington Post reported Wednesday that Mueller’s investigation into Russia’s interference in the 2016 election has expanded to include an examination of whether the president attempted to obstruct justice.

“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 tweeted early Thursday. 

He later added: ‘You are witnessing the single greatest WITCH HUNT in American political history - led by some very bad and conflicted people! #MAGA"

Daniel Coats, the current director of national intelligence, Adm. Mike Rogers, the head of the National Security Agency and Richard Ledgett, Rogers’ departed deputy, all agreed to participate in interviews with Mueller’s investigators as early as this week, five anonymous sources briefed on the requestes told the Post.

It wasn’t immediately clear if others have been questioned by the FBI.

Investigators are also reportedly looking for proof of possible financial crimes committed by Trump associates.

“The FBI leak of information regarding the President is outrageous, inexcusable and illegal,” Mark Corallo, a spokesman for Trump’s personal attorney, Marc Kasowitz, said Wednesday.

Mueller’s office declined to comment.

The investigation into obstruction of justice into Trump reportedly started days after former FBI Director James Comey’s May 9 ouster.

According to the Post, a March 22 exchange among Trump, Coats, and CIA Director Mike Pompeo, alone in the White House, is of potential interest to Mueller. Coats reportedly said that Trump had asked him if he could intervene with Comey to get the FBI to back off focus on former national security adviser Michael Flynn as part of the Russia probe.