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.

The special counsel is investigating any potential ties between Russians and Trump campaign associates.

There are several links between the Trump campaign and WikiLeaks, including private messages on Twitter between Donald Trump Jr. and WikiLeaks and outreach from the chief executive of Cambridge Analytica to WikiLeaks founder Julian Assange. Mueller's team is looking into whether the communications were ever intended as a coordinated effort to help with Russia's 2016 election meddling.

The President has repeatedly denied any collusion.

Stone has also denied ever receiving anything from WikiLeaks.

"I never received any material from them at all," he said last week. "I never received any material from any source that constituted the material ultimately published by WikiLeaks. ... This will be an impossible case to bring because the allegation that I knew about the (WikiLeaks) disclosures beyond what Assange himself had said in interviews and tweets, or that I had and shared this material with anyone in the Trump campaign or anyone else, is categorically false."