Special counsel Robert Mueller’s team of investigators probing the 2016 U.S. presidential race has taken an interest in the finances of Roger Stone, President Trump’s former election campaign adviser and longtime confidant, CNN reported Thursday.

Mr. Mueller’s team has questioned some of Mr. Stone’s associates, seeking information about his finances, including his tax returns, multiple people familiar with the matter told CNN, the network said.

The reported interest in Mr. Stone’s finances comes on the heels of the special counsel’s office recently subpoenaing at least two of his associates as Mr. Mueller’s team continues to investigate the 2016 race and possible collusion between the Trump campaign and Russia.

A former aide to Richard Nixon, Mr. Stone, 65, served as an early adviser to the Trump election campaign prior to parting ways in 2015. He’s faced scrutiny over his conduct closer to Election Day, however, including his repeated comments about WikiLeaks, the antisecrecy website that published documents damaging to Mr. Trump’s opponent in the race’s final weeks.

The U.S. intelligence community has concluded that Russian state-sponsored hackers sourced the documents published by WikiLeaks in 2016, including emails stolen from the personal account of John Podesta, the chairman of Democratic nominee Hillary Clinton’s presidential campaign. Mr. Stone claimed during the race of being in touch with WikiLeaks publisher Julian Assange, and he successfully predicted the website’s release of damaging Clinton documents.

“There’s no inappropriate activity pertaining to Russian collusion,” Mr. Stone said Thursday. “I obtained nothing from WikiLeaks or Julian Assange. I never passed anything on to WikiLeaks or Julian Assange,” he told CNN.

The special counsel’s office recently subpoenaed Mr. Stone’s accountant, John P. Kakanis, and a social media consultant he hired during the 2016 campaign, Jason Sullivan, as part of its Russia probe, news outlets reported this month.

At least eight of Mr. Stone’s current or former associates have been contacted by the special counsel’s office, he told NBC over the weekend.

Mr. Stone has yet to be contacted by the special counsel’s office, he told CNN.

“Having come up empty handed in his search for Collusion with Russian or Wikileaks Mueller now seeks to conjure some bogus financial or tax matter to silence and bankrupt,” Mr. Stone wrote said in a social media post.

The special counsel’s office has a brought criminal charges against some 22 individuals and entities since getting off the ground least May. Five people have pleaded guilty so far, including three people tied to Mr. Trump’s 2016 presidential campaign: Michael Flynn, George Papadopoulos and Rick Gates.

Mr. Trump, on his part, has repeatedly decried the probe as a “witch hunt.”

The special counsel’s office declined to comment for CNN’s report.