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(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.

02:34 - Source: CNN
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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.