Washington (CNN) Roger Stone associate Andrew Miller will have to testify to a grand jury in special counsel Robert Mueller's investigation or will go to jail, a three-judge panel at the DC Circuit Court of Appeals ruled Tuesday morning, in an endorsement of Mueller's authority as a prosecutor.

The court agreed with a trial-level judge's ruling that Miller should be held in contempt of court and jailed for refusing to testify under a grand jury subpoena from Mueller.

The appeals court said that Mueller was legally appointed by Deputy Attorney General Rod Rosenstein as special counsel in May 2017. Mueller has conducted a grand jury investigation that is within his legal authority, the court wrote in its opinion.

Miller's challenge had tested whether Mueller could call witnesses to a grand jury and take other prosecutorial steps. At least one other opponent in Mueller's investigation, the Russian company Concord Management and Consulting, had closely watched Miller's challenge, as a possible relief from its own criminal charge, which the company continues to fight.

The DC Circuit judges said that Rosenstein, who became acting attorney general following Attorney General Jeff Sessions' recusal from the Russia probe, had the ability to appoint Mueller under the Constitution.

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