A witness has reportedly been issued with a writ requiring them to turn over all documents relating to Donald Trump and some of his closest advisers, by the grand jury investigating allegations of Russian collusion in the 2016 US presidential election.
It is said to ask for all documentation between Mr Trump and some of his core team, including the US president's former chief strategist Steve Bannon and ex-White House communications director Hope Hicks.
It also asks for communications involving Mr Trump's former campaign chairman Paul Manafort, who is already facing a variety of charges, including conspiracy to launder money, filing false tax returns and failing to register as a foreign agent for lobbying work for the pro-Kremlin Ukrainian government of former President Viktor Yanukovych. His trial will be heard in September.
Mr Mueller's team have also demanded to see communications from six other people.
Significantly, the subpoena will seek communications from before Mr Trump announced his candidacy.
Former CIA official Ned Price said the revelation showed Mr Mueller was treating Mr Trump’s "entire senior campaign team like a criminal enterprise.”
This is, with just a few exceptions, essentially the entire senior campaign team. Mueller is treating it like a criminal enterprise. https://t.co/YziEQUoMkS
— Ned Price (@nedprice) March 5, 2018
Mr Price also claimed it was significant that Roger Stone was named in the document.
The former campaign adviser is reportedly the focus of the investigation into Mr Trump’s contacts with WikiLeaks during his election campaign.
“Several targets never took up a position in the White House, suggesting this particular subpoena isn’t about obstruction," Mr Price said.
He added: "The inclusion of Roger Stone, who had foreknowledge of the Podesta leaks, indicates this could be about potential collusion."
It comes after the focus of the investigation recently turned to Mr Trump’s alleged attempts to push Attorney General Jeff Sessions to resign.
The investigation will reportedly examine whether this could be part of a pattern of obstructing justice in the investigation.
Mr Mueller has been asking witnesses about Mr Trump’s comments from July to August 2017, when the President was tweeting insults about Mr Sessions, sources told The Washington Post.