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 will appear in court later in the city of Fort Lauderdale.
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 outlet says.
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.
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 senior Trump campaign officials to inquire about future releases” by WikiLeaks.
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 a state-authorised relay of cyber attacks and fake news stories planted on social media.
Mr Trump has branded the Mueller inquiry a political “witch-hunt”. The Kremlin has always denied meddling in the US election.
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.
“I would chalk this up to an effort to silence me.”
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 Wikilieaks 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 longtime 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 notorious 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?
A long-time friend of Donald Trump, Roger Stone has worked on Republican political campaigns since the 1970s.
The 66-year-old favours three-piece suits and reportedly refuses to 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.