President Donald Trump on Thursday claimed, without offering any evidence, that "the inner workings of" special counsel Robert Mueller's "investigation are a total mess."

"They have found no collusion and have gone absolutely nuts. They are screaming and shouting at people, horribly threatening them to come up with the answers they want," Trump said in a tweet, one of four blasting Mueller and his team.

"They are a disgrace to our Nation" and "gang of Democrat thugs."

Trump's remark about the "inner workings" of the Mueller probe come amid ongoing controversy over the president's firing last week of Attorney General Jeff Sessions.

Trump named Sessions's chief of staff, Matthew Whitaker, as acting attorney general.

Whitaker before joining the Justice Department as Sessions's chief of staff had been a critic of the Mueller investigation. He now has oversight over that probe and power to fire Mueller for cause.

Trump's Twitter tantrum came as Democrats in Congress and Republican Sen. Jeff Flake of Arizona called for legislation that would protect the former FBI Director Mueller from being fired before his investigation is finished.

Trump has been angered since last year over Mueller's probe into whether Trump's presidential campaign colluded with the Russians to interfere in the 2016 election, and into whether the president obstructed justice by trying to impede that inquiry.

Trump has repeatedly denied both claims. He also has highlighted the fact that a number of Mueller's prosecutors have been registered Democrats and contributed money to Democrats, although he does not mention that Mueller himself is a Republican.

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The president had long blamed Sessions for Mueller's appointment, which was set in motion when Sessions recused himself from overseeing investigations into Russian interference because of his own contact with Russians during the campaign.

Two days before Trump teed off on Mueller, NBC News reported that the president's legal team was close to finishing a series of written answers Mueller's team has posed to Trump.

House Minority Leader Nancy Pelosi, D-Calif., criticized Trump for his latest tweets, saying the president "continues to wage an all-out campaign to obstruct the Mueller investigation."

And Pelosi accused Trump of putting Whitaker in charge of the Justice Department "for one purpose — to end the investigation."

Democrats, and some Republicans, have said that Whitaker could not legally be appointed acting attorney general because he was being elevated from a position that did not require Senate approval. The Justice Department disputed that argument in an opinion released Wednesday.

Trump called that "a very strong opinion" in an interview Wednesday with The Daily Caller.

In that same interview, the president linked Whitaker to Trump's longstanding critique of the Mueller investigation.

Whitaker, Trump said, "is just somebody that's very respected."

"I knew him only as he pertained, you know, as he was with Jeff Sessions," Trump told The Daily Caller. "And, you know, look, as far as I'm concerned this is an investigation that should have never been brought. It should have never been had.

"It's something that should have never been brought. It's an illegal investigation. And you know, it's very interesting because when you talk about not Senate confirmed, well, Mueller's not Senate confirmed."