(CNN) Special counsel Robert Mueller's team has been probing Roger Stone's finances as it summons a series of witnesses to gather more information about one of President Donald Trump's longtime advisers, according to people familiar with the situation.

Mueller's team has questioned associates about Stone's finances, including his tax returns.

The special counsel recently subpoenaed John P. Kakanis, who has worked as Stone's assistant and has insight into some of Stone's business deals, according to a person familiar with the situation. Reuters first reported on the subpoena.

Stone has insisted he played no role in colluding with Russians. Stone also said he has not been contacted by the special counsel's office.

The interest in Stone's finances could be tied to Mueller's charge of investigating Russian meddling in the 2016 election and potential collusion, though another possibility is Mueller is pursuing something unrelated that turned up in the course of the investigation.

"The normal thing for a prosecutor in the course of your investigation is if you come across something that is itself worthy of criminal investigation, you don't turn a blind eye to it. You investigate," said CNN legal analyst Michael Zeldin, a former assistant to Mueller.

Whatever the reasoning, the probe into Stone's finances should give him cause for concern, Zeldin said.

Stone appears to recognize that.

"The special counsel having found no evidence or proof whatsoever of Russian collusion, trafficking in allegedly hacked emails with WikiLeaks or advance knowledge of the publication of (then-Clinton campaign chair John) Podesta's emails now seems to be combing through every molecule of my existence including my personal life, political activities and business affairs to conjure up some offense to charge me with either to silence me or induce me to testify against the President," Stone told CNN. "I have no intention of being silenced or turning my back on President Trump."

The questions about Stone's finances have some of his allies claiming that Mueller has overstepped his mandate and is simply looking to take down one of the President's longtime allies.

"They are trying to go after Roger's finances, to get another ridiculous indictment that has nothing to do with Russia," said Sam Nunberg, a former Trump campaign aide who worked with Stone on the early stages of Trump's presidential bid and has been questioned by Mueller's team as well as the grand jury.

Nunberg pointed to the charges Mueller's team has brought against former Trump campaign chairman Paul Manafort for alleged financial crimes unrelated to his work on the Trump campaign. He also noted the raid on the home, office and hotel room of Michael Cohen, Trump's longtime personal attorney. The raid and related criminal investigation are being carried out by the Southern District of New York, but were referred there by Mueller's team.

"If they want to continue to go after non-related financial matters where they're picking apart associates of the President, then by all means," Nunberg said. "Because they continue to delegitimize themselves and whatever they ultimately release in their findings."

But Mueller's mandate when he was first appointed specifically gives him the authority to investigate " any matters that arose or may arise " directly from the Russia probe.

So far, Mueller's team has contacted at least eight of Stone's current or former associates, Stone told NBC News on Sunday. He subsequently told CNN "they are all young people who have no knowledge about my personal, political or business activities."

Some of those names are still unknown, and Stone declined to provide them. In addition to subpoenaing Kakanis, Mueller's team subpoenaed Jason Sullivan, Stone's former social media adviser, to appear before the grand jury.

"Jason has done nothing wrong and done everything right, and he's terrific at what he did," said Knut Johnson, Sullivan's lawyer.

The special counsel's office declined to comment. A lawyer for Kakanis did not respond to requests for comment.

The special counsel's office has already interviewed Nunberg and former Trump campaign adviser Michael Caputo. Both men were pressed for information regarding Stone in their interviews, according to people familiar with the questioning.

Stone has come under public scrutiny, in part, because of a prescient prediction during the 2016 campaign. In a now-infamous tweet, Stone predicted trouble for Podesta, weeks before WikiLeaks began releasing thousands of Podesta's emails. Stone denies having any advance knowledge of the Podesta leaks.

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"I sleep well at night because I know what I have and have not done," Stone told CNN. "There's no inappropriate activity pertaining to Russian collusion. I obtained nothing from WikiLeaks or Julian Assange. I never passed anything on to WikiLeaks or Julian Assange."

As Mueller's team investigates Russian interference in the 2016 election and potential collusion between Trump associates and the Russian government, people familiar with the probe have expected Mueller to eventually bring charges related to the Russian hacking of the DNC and Democratic officials.

A self-proclaimed "dirty trickster," Stone cut his teeth in politics on Richard Nixon's 1972 re-election campaign and made a career of political consulting and lobbying.

Even after he officially parted ways with Trump's presidential campaign in its early days, Stone remained a staunch supporter and friend of Trump's. During the campaign, he launched a pro-Trump super PAC called Committee to Restore America's Greatness.

Now, the interest in Stone's finances has created a new sense of alarm among his associates.

"The special counsel is going to be woefully disappointed when they find out they have absolutely no proof and Roger Stone did not collude with Russians. But that will not stop them from creating a crime simply because he's close to the President of the United States," Caputo said in an interview. "This is a punishment strategy."