Special counsel Robert Mueller's probe has expanded to include a filmmaker who interviewed Roger Stone for a documentary about alternative media and censorship called "Sensational" in 2017.

David Lugo told NBC News he testified before a Washington grand jury in October about Stone and Stone's alleged backchannel to WikiLeaks, comedian and activist Randy Credico.

Mueller's team is investigating possible collusion between the Trump campaign and Russia, including whether the campaign had advance knowledge that hacked Democratic emails would be released by WikiLeaks.

Lugo becomes one of nine Stone associates known to have been questioned in the Mueller probe.

Lugo said that in May 2017, about an hour before filming the interview with Stone, while standing in line at a New York Starbucks, Credico told him, "I was the guy who connected Roger to the WikiLeaks stuff."

Lugo said Credico subsequently tried to get his production company to pay for Credico to fly to London to interview WikiLeaks founder Julian Assange for the film, which is nearing release.

"He was very much trying to get to London to have a face-to-face with Assange," Lugo said.

Credico has been adamant that he did not serve as a go-between between Assange and Stone in the lead-up to the 2016 presidential election. He told NBC News the first time he ever spoke to Assange was on his local New York radio show on Aug. 25, 2016, after Assange had started leaking hacked emails damaging to then-candidate Hillary Clinton.

Credico notes that the radio conversation occurred after Stone began predicting another WikiLeaks dump of emails that turned out to be the "October surprise," which included the emails of Clinton campaign chairman, John Podesta.

Stone disputed Credico's argument and said the date of Credico's interview with Assange is irrelevant because Credico was getting information about WikiLeaks from one of Assange's lawyers, whom Credico has known for years, not Assange himself.

"Randy's source is a lawyer who works for WikiLeaks who [has been] a professional associate of his for 30 years and who is the person who later gets Credico his interview," he said. NBC News reached out to the lawyer for WikiLeaks but she did not immediately respond.

Credico called Lugo "an alt-right fanatic" on Wednesday, and said Lugo fabricated the story about the Starbucks revelation at Stone's behest.

David Lugo says he took this photo of Roger Stone with Randy Credico in Stone's apartment in New York. David Lugo

Lugo, for his part, said Credico has been harassing him for months, and that while he voted for President Donald Trump, and has appeared with Alex Jones on the alt-right radio show Infowars, he is not a fanatic.

Lugo told NBC News two FBI agents came to his Philadelphia home unannounced on Oct. 12. He said he joked with them that he was expecting them months before. Lugo said the agents asked him to testify before a grand jury about what he knows about Stone, Credico and Assange.

Lugo said he provided investigators with all the texts, emails and Facebook messages he has exchanged with Stone and Credico since he met them in 2017.

A spokesman for the special counsel's office declined to comment.