Michael Anderson was freed in October when the state dismissed criminal charges that had alleged a murder conspiracy plot targeting judges and state troopers.
Cox and several members of the Alaska Peacekeeper Militia recently pleaded not guilty to federal charges that they conspired to possess illegal firearms. Anderson was never charged in the federal case.
Assistant U.S. Attorney Steve Skrocki tells the Fairbanks Daily News-Miner that Anderson will have an initial court appearance Wednesday and bail will be discussed.
Anderson was brought Tuesday to the Fairbanks Correctional Center.
The newspaper says a material witness warrant is a tool prosecutors sometimes use in an effort to compel someone to testify in a criminal case when the person is not cooperating with a subpoena.