U.S. Special Counsel Robert Mueller just called in one more aide to Trump adviser Roger Stone for questioning, Reuters reported Friday.

Investigators probing possible Russian interference in the 2016 election served a subpoena to John Kakanis, 30, who has worked as a driver and political operative for Stone, Reuters reported, citing two people familiar with the situation.

This is the second subpoena issued to a person linked to Stone in as many weeks. The questioning of multiple aides to Stone aides is the latest sign Mueller’s investigators are focusing in on the infamous political operative who first emerged onto the American national political scene during the Watergate scandal in the 1970s.

In an emailed statement to VICE News, Stone denied any wrongdoing.

“Although I operated independent of the Trump campaign all of my activities to promote the nomination and election of Donald Trump were completely legal,” Stone wrote.

Last week, Mueller’s investigators also subpoenaed Stone’s former social media director, Jason Sullivan, Sullivan’s attorney told CNN on Wednesday.

Two other former aides to the Trump campaign, Michael Caputo and Sam Nunberg, have said they were asked about Stone during their interviews with Mueller’s investigators, according to Politico.

Stone’s public comments during the campaign have prompted questions over whether he may have had real-time knowledge of the alleged Russian hack of computer systems belonging to the Democratic National Committee. The contents were later published online by a website called WikiLeaks.

One person who spoke to Reuters said Kakanis was questioned by the FBI on the topics of possible Russian interference in the 2016 presidential election, WikiLeaks, its founder Julian Assange, and the hacker or hackers who call themselves Guccifer 2.0.