Special counsel Robert Mueller has issued a subpoena to an assistant of longtime Trump confidant Roger Stone, according to a report Friday.

John Kakanis, who worked for Stone in several capacities including as a driver and an accountant, was subpoenaed by Mueller and has been questioned by the FBI about Russian interference in the 2016 election and WikiLeaks founder Julian Assange, among other things, Reuters reports.

Earlier this week it was reported that Republican social media consultant Jason Sullivan, who conducted work on behalf of Stone’s Committee to Restore America’s Greatness, was served two subpoenas from the special counsel. One requests Sullivan make an appearance in front of a grand jury for the special counsel and another demands documents and electronic information.

Stone attracted scrutiny after he appeared to anticipate a WikiLeaks document release, saying in 2016 on Twitter that Hillary Clinton campaign chairman John Podesta's "time in the barrel" would soon occur, after which Podesta's hacked emails were published by WikiLeaks.

President Trump's campaign aides Michael Caputo and Sam Nunberg have said prosecutors for the special counsel’s team asked them about Stone.

Stone has denied ever having any early-access knowledge from WikiLeaks about the release of Podesta's emails. Rather, he claimed the statement was based on his own investigations of Podesta.

WikiLeaks is well-known for publishing leaked secrets on its website. Among its controversial publications, the website has made public stolen emails from Democratic officials during the 2016 campaign as well as documents on CIA hacking tools.

A U.S. intelligence community assessment determined with "high confidence" last January that WikiLeaks was used by Russian intelligence to release information as part of an effort to elect Trump. WikiLeaks denies this assertion.