(CNN) Special counsel Robert Mueller has subpoenaed Randy Credico to testify before a grand jury next month, according to Credico's attorney.

Martin Stolar told CNN he received a subpoena Thursday and Credico intends to comply and testify on September 7.

Credico, a comedian and radio show host, will likely meet with Mueller's team for a voluntary interview first. Credico previously declined a request from Mueller's team for a voluntary interview.

Roger Stone claimed Credico was his back channel to WikiLeaks founder Julian Assange during the 2016 campaign.

Stolar said it's unclear exactly what Mueller's team wants to question Credico about.

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