Roger Stone associate will testify later this week as part of Mueller probe -- which apparently hasn't ended
Roger Stone speaks to ABC News (screen grab)

An associate of Roger Stone has agreed to testify before a grand jury Friday as part of special counsel Robert Mueller's investigation.

Mueller resigned from the Department of Justice during a news conference Wednesday, but portions of his sprawling investigation of Russian election interference remain active despite his departure, reported CNN.

Stone associate Andrew Miller had challenged Mueller's authority since last summer to obtain information related to the Republican activist and WikiLeaks, and Judge Beryl Howell held him in contempt.

Miller lost his appeal during a hearing held around the same time as Mueller's news conference.

Mueller prosecutor Aaron Zelinsky, who has handled the case against Stone, notified Miller's attorneys why his testimony was still needed, and he agreed to testify later this week.

Zelinsky declined to say what part of the Justice Department he's working with, and he declined to comment on Mueller's public statement.