US President Donald Trump has attacked Robert Mueller's investigation into Russian interference in the 2016 US presidential election, calling it a "total mess" and "absolutely nuts".
He tweeted on Thursday that investigators were "threatening" people to provide "the answers they want".
"They are screaming and shouting at people," Mr Trump wrote, adding that no collusion between his election campaign and Russia had been found.
He says the inquiry is a "witch hunt".
The US president has previously said the special counsel investigation led by Mr Mueller, who is a highly regarded former head of the FBI and a Republican, was unfair and dominated by "hardened Democrats".
In Mr Trump's latest tweets, he describes Mr Mueller as "conflicted" and says that those involved in the long-running probe "are a disgrace to our nation".
The inner workings of the Mueller 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. They are a disgrace to our Nation and don’t...— Donald J. Trump (@realDonaldTrump) November 15, 2018
....care how many lives the ruin. These are Angry People, including the highly conflicted Bob Mueller, who worked for Obama for 8 years. They won’t even look at all of the bad acts and crimes on the other side. A TOTAL WITCH HUNT LIKE NO OTHER IN AMERICAN HISTORY!— Donald J. Trump (@realDonaldTrump) November 15, 2018
The president later tweeted that Mr Mueller and "his gang of Democrat thugs" were destroying people, and blamed social media tech giants for unfavourable coverage towards his administration and the Republicans.
"Check out how biased Facebook, Google and Twitter are in favour of the Democrats," he wrote.
It is unclear what was behind his latest comments and Mr Trump did not provide any evidence for his claims.
Fears of an impending storm
Analysis by Anthony Zurcher, BBC Washington
Donald Trump is back on the attack against Robert Mueller's special counsel investigation. After a brief mid-term interlude that included dire warnings about refugee caravans in Mexico and mix-and-match endorsements of Republican candidates, the looming Russia investigation is back on the president's mind - and his Twitter feed.
Mr Trump may simply be reflecting the growing sense that Mr Mueller is poised for new action after a pre-election quiet period. Rumours and reports have swirled of impending indictments and late nights at the special counsel's office.
There's also the possibility that the president knows something new - either passed along from his lawyers or allies under investigation, or from his newly appointed acting attorney general, who the New York Times referred to in September as the White House's "eyes and ears" in the justice department.
Whatever the explanation, Mr Trump has appeared in a sour mood of late, lashing out at the media and, according to background sources, considering changes to his top staff. There is the feeling in Washington of an impending storm, of threats to the presidency from the outside and within.
It's been a quiet few days, but that seems certain to change.
It comes just a week after the US president forced the resignation of Jeff Sessions, the US Attorney General who was an early supporter of Mr Trump.
Mr Sessions had earlier voluntarily removed himself from the Russia probe after Democrats accused him of failing to disclose contacts with the Russian ambassador during his Senate confirmation hearing.
Following this decision, Mr Trump told the New York Times: "Sessions should have never recused himself, and if he was going to recuse himself, he should have told me before he took the job and I would have picked somebody else."
Mr Sessions' replacement in charge of the Department of Justice, Matthew Whitaker, now has the power to sack Mr Mueller or end the investigation.
Mr Whitaker, who has been appointed acting attorney general, has been open about his concerns over the scope of Mr Mueller's inquiry.
What is behind the Russia investigation?
In 2016, US intelligence agencies concluded that Russia had used a state-authorised campaign of cyber attacks and fake news stories planted on social media in an attempt to turn the election against Democrat candidate Hillary Clinton.
A team of investigators led by Mr Mueller is looking into whether anyone from Mr Trump's campaign colluded in the effort.
It has been established that senior members of Mr Trump's team met Russian officials, while several of these meetings were not initially disclosed.
The president's son, Donald Trump Jr, met a Russian lawyer during the campaign who was said to have "dirt" on Mrs Clinton, and adviser George Papadopoulos has admitted lying to the FBI about meetings with alleged go-betweens for Russia.
Four people connected with Mr Trump's campaign and presidency have been charged and further indictments could be issued.
However the US president denies any wrongdoing and no solid evidence has emerged to implicate him.