Washington (CNN) Jerome Corsi, a prominent conspiracy theorist with ties to Roger Stone, was subpoenaed to testify before a federal grand jury Friday as part of special counsel Robert Mueller's Russia investigation, according to Corsi's attorney.

But Corsi's appearance was postponed after he met with Mueller's investigators for hours Thursday afternoon for a voluntary interview ahead of his scheduled grand jury appearance. His attorney, David Gray, would not comment on what Corsi was asked about during his voluntary interview or why the grand jury appearance was postponed. It may be rescheduled for a later date, he said.

"We are going to be fully cooperative. We have nothing to hide," Gray told CNN when the news of the subpoena was first reported by The New York Times earlier this week. "While we believe the focus will be on his communications with Roger Stone, that is an assumption."

Corsi had been one of the latest in a string of Stone associates to be summoned by Mueller's team. Randy Credico, a comedian who's another former associate of Stone's, was also slated to appear before the grand jury Friday and did so.

Stone, a longtime political adviser to President Donald Trump, attracted scrutiny in 2016 after claiming to have a back channel to WikiLeaks and appearing to predict that the site would release damaging information related to Hillary Clinton during the presidential campaign. Stone has since said he had no contact with Julian Assange, the founder of WikiLeaks. Meanwhile, US intelligence agencies concluded that Russian intelligence had hacked Democratic targets and used WikiLeaks to spread that material online.

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