Washington (CNN) Roger Stone's potential ties to WikiLeaks and its founder, Julian Assange, are being investigated by special counsel Robert Mueller, The Wall Street Journal reported Monday, citing a person familiar with the matter.

Stone served as an adviser on President Donald Trump's presidential campaign, and according to The WSJ report, Stone said in an email on August 4, 2016, that he had "dined with Julian Assange last night."

However, Stone has denied ever meeting Assange.

In a text exchange on Friday before the WSJ report, Stone said he "never met or spoke with Assange ever," and Stone told The Journal the contents of the email were "said in jest."

Stone also noted that his passport showed that he did not leave the country in 2016.

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