Political strategist Roger Stone, a long-time ally of President Trump, has been arrested in Florida, charged with seven counts in the Mueller probe.
Mr Stone, 66, appeared in court in the city of Fort Lauderdale.
The indictment includes one count of obstruction of an official proceeding, five counts of false statements, and one count of witness-tampering.
The charges are linked to an alleged Russian-led hack into the emails of Democratic Party officials.
The information contained in the emails was released by Wikileaks during the 2016 US presidential election campaign.
CNN reports that FBI agents arrested Mr Stone in a pre-dawn raid on Friday. One pounded on the door and said, "FBI, open the door," the US cable network says.
After news of his old friend's arrest, President Donald Trump slammed the Russia investigation once again on Twitter, calling it the "Greatest Witch Hunt in the History of our Country!"
Greatest Witch Hunt in the History of our Country! NO COLLUSION! Border Coyotes, Drug Dealers and Human Traffickers are treated better. Who alerted CNN to be there?— Donald J. Trump (@realDonaldTrump) January 25, 2019
What exactly is Stone charged with?
The special counsel has accused Mr Stone of attempting to obstruct the investigation into Russian meddling by:
- Lying to the House Intelligence Committee about what transpired between him and Wikileaks
- Lying to the committee about records of his interactions with Wikileaks
- Witness tampering, for trying to keep his Wikileaks intermediary from telling the truth to Congress
The indictment does not say Mr Stone committed any criminal activity during the election.
How did Roger Stone respond?
He was released on a $250,000 (£189,000) bond, but with restrictions allowing him to travel only for court appearances in Florida, Washington DC and New York.
After his court appearance, Mr Stone spoke on the phone to conspiracy theorist Alex Jones on his radio programme Infowars.
"I intend to fight for my life," Mr Stone said as he launched a fundraiser for his legal battle. "There's no circumstance under which I would plead guilty to these charges.
"There's no circumstance under which I would bear false witness to the president."
He added: "Once again, there is no evidence of Russian collusion, Wikileaks collaboration and I'm not charged with doing anything inappropriate or illegal to assist in his [Mr Trump's] election."
Mr Stone then emerged from court grinning broadly while flashing a Nixonian double V-sign and wearing a blue Ralph Lauren polo shirt.
He maintained his innocence though was at times inaudible because of loud boos and chants of "Lock him up!"
Mr Stone said he would formally plead not guilty to the charges in a Washington DC court next week.
He declared himself the target of "a politically motivated investigation" and criticised the manner of his arrest.
"At the crack of dawn, 29 FBI agents arrived at my home with 17 vehicles with lights flashing. They terrorised my wife [and] my dogs."
What links Stone to the email hack?
Hillary Clinton's 2016 campaign chairman John Podesta, who was targeted in the email hack, accused Mr Stone of knowing about it beforehand.
More than a month before the emails were made public, Mr Stone tweeted it would soon be Mr Podesta's "time in a barrel", which critics say indicates he already knew about the hack.
Mr Stone has denied having advance knowledge of the cyber-breach and any knowing contact with Russian agents.
On Friday, Mr Podesta referenced the cryptic tweet in a post about Mr Stone's indictment, noting it was "Roger's time in the barrel".
According to investigators, Mr Stone said he had "communicated" with Wikileaks founder Julian Assange before the emails' release and had described the contact as "perfectly legal".
The indictment accuses the lobbyist of lying to the House Intelligence Committee about what passed between him and Wikileaks, and of falsely claiming not to have records of it.
Mr Stone is said to have spoken to senior Trump campaign officials - who are not identified - about "organization 1" [believed to be Wikileaks] "and information it might have had that would be damaging to the Clinton Campaign".
Prosecutors say Mr Stone was also contacted by unnamed "senior Trump campaign officials" to inquire about future Wikileaks releases.
They also allege he made false statements about his communications with conservative writer and conspiracy theorist Jerome Corsi regarding the Wikileaks email leak.
The indictment points to conversations about the planned Wikileaks release to the public between Mr Stone and "Person 1" and "Person 2", who are not identified by name.
Lawyers for Mr Corsi confirmed he is "Person 1" in the indictment on Friday, adding that they believe he is not accused of any wrongdoing in the Mueller investigation.
His campaign activities have long been under scrutiny by Special Counsel Robert Mueller, who is investigating allegations of collusion between Russia and the Trump campaign.
In 2016, US intelligence agencies concluded that Russia had tried to turn the election in Mr Trump's favour through state-sponsored cyber-attacks and fake news stories spread on social media.
The Mueller investigation has also focused on Mr Stone's communications with Twitter persona Guccifer 2.0, which US intelligence officials say is a front for Russian military intelligence.
Mr Trump has repeatedly assailed the Mueller inquiry. The Kremlin has always denied meddling in the US election.
The president's counsel, Jay Sekulow, said: "The indictment today does not allege Russian collusion by Roger Stone or anyone else. Rather the indictment focuses on alleged false statements made to Congress".
Tom Perez, chair of the Democratic National Committee, accused the Trump campaign of being "a willing and active participant in a conspiracy with Russia and Wikileaks" in a statement.
Mr Perez added that "there are more conspirators yet to be held accountable - and at least one of them is named Donald Trump".
Mr Stone has repeatedly said in interviews that he expected to be indicted. In May he told NBC's Meet the Press: "It is not inconceivable now that Mr Mueller and his team may seek to conjure up some extraneous crime pertaining to my business, or maybe not even pertaining to the 2016 election."
The missing link
Through past indictments Robert Mueller has been building the case that Russians were behind the hacking of senior Democratic and Hillary Clinton campaign officials. He alleges they then used Wikileaks to inject damaging information they garnered into the US political bloodstream at a time most harmful to Democrats.
What the special counsel's office had yet to allege is that there were any ties between Wikileaks and the Donald Trump presidential campaign.
On Friday Mr Mueller charged long-time Trump adviser and confidant Roger Stone with being that missing link, citing communications between Mr Stone and "senior" Trump campaign officials before and after the Wikileaks documents were released.
The move marks a significant development in this nearly two-year legal drama. For the first time a member of Mr Trump's inner circle, albeit an informal one, is facing charges directly related to Russian election meddling - the central thrust of the special counsel's investigation.
Mr Stone has denied suggestions of criminal misconduct. But the man who has boasted of being a master of the political dark arts, and who seemed to crave the notoriety that went along with such claims, now finds himself directly in the centre of a withering legal spotlight.
Who's been charged in the Mueller probe?
Mr Stone is the 34th person to be charged as part of the Mueller investigation. Those indicted include 12 Russian military officers and 13 Russian nationals accused of leading a campaign to interfere in the US election.
Another three Russian entities, including the Internet Research Agency "troll farm", have also been charged.
Several people connected directly with Mr Trump have been indicted, including his former national security adviser Michael Flynn, and Paul Manafort, the former chairman of his election campaign, who is in jail.
Mr Trump's former personal lawyer Michael Cohen and former campaign adviser George Papadopoulos have also been charged.
Who is Roger Stone?
An old friend of Donald Trump, Roger Stone has worked on Republican political campaigns since the 1970s.
A self-proclaimed Republican "dirty trickster", he favours three-piece suits and reportedly does not wear socks.
He began his career working on Richard Nixon's 1972 re-election bid, and has a tattoo of the 37th president across his shoulder blades.
Mr Stone became embroiled in the Watergate scandal after congressional hearings revealed that he hired an operative to infiltrate the campaign of George McGovern, Mr Nixon's Democratic opponent.
He went on to work for Ronald Reagan's successful presidential campaigns in 1980 and 1984, and advised George HW Bush in his bid for the White House in 1988.
Mr Stone published a book, The Making of the President 2016, after helping Mr Trump to power.