WASHINGTON (CNN) -- President Donald Trump slammed special counsel Robert Mueller's expanding probe on Thursday, calling the inquiry into whether he obstructed justice a "phony story" and "the single greatest WITCH HUNT in American political history."

Trump, in a series of morning tweets, reverted back to the tone that won him the White House, slamming the investigation a day after he called for unity in the wake of a shooting at a Republican congressional baseball practice that injured several people, including House Majority Whip Steve Scalise.

"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.

"You are witnessing the single greatest WITCH HUNT in American political history - led by some very bad and conflicted people! #MAGA," Trump tweeted.

The Washington Post first reported Wednesday that Mueller's probe was widening to include looking into whether Trump obstructed justice.

Mueller's investigators have asked for information and will talk to Director of National Intelligence Dan Coats and National Security Agency Director Adm. Mike Rogers, a source familiar with the matter told CNN. They will also seek information from retired NSA Deputy Director Richard Ledgett, the source said, because Ledgett wrote a memo documenting some of Rogers' conversations with Trump.

The inquiries appear to be part of a fact-finding mission and it would ultimately be up to Mueller to decide whether there is enough evidence to recommend pursuing charges on any part of the investigation.

A spokesman for Trump's private attorney declined to respond to details of the widening probe Wednesday, instead slamming the leaked information as "outrageous, inexcusable and illegal."

Trump has regularly turned to Twitter to blast reports about his alleged connections to Russia or the inquiries into possible collusion between the Trump campaign and Russian operatives.

But Thursday's comments come less than 24 hours after the President urged unity after the shooting.

"We may have our differences, but we do well in times like these to remember that everyone who serves in our nation's capital is here because above all they love our country," Trump said at the White House.