A federal appeals court on Tuesday rejected a challenge to the appointment of Special Counsel Robert Mueller, after a former assistant to Trump political adviser Roger Stone brought the case following his refusal to comply with a grand jury subpoena.

Stone associate Andrew Miller refused to testify before the grand jury as part of the Russia investigation, claiming Mueller was illegitimately appointed as special counsel. Miller claimed that Mueller’s appointment was “unlawful” because he was appointed by Deputy Attorney General Rod Rosenstein, rather than former Attorney General Jeff Sessions.

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The U.S. Court of Appeals for the District of Columbia Circuit on Tuesday affirmed the validity of Mueller’s appointment to investigate Russian meddling and potential collusion with Trump campaign associates in the 2016 presidential election.

“Because the Special Counsel is an inferior officer, the Deputy Attorney General became the head of the Department [of Justice] by virtue of becoming the Acting Attorney General as a result of a vacancy created by the disability of the Attorney General [Jeff Sessions] through recusal on the matter, we hold that Miller’s challenge to the appointment of the Special Counsel fails,” the court wrote in its decision. “Accordingly, we affirm the order finding Miller in civil contempt.”

At this point, Miller has the ability to ask the Supreme Court to weigh in on the constitutionality of the special counsel appointment process, which is now overseen by Attorney General William Barr.

Prior to the decision, a lower court also held Miller in contempt. It is unclear whether Miller will now have to appear to testify before the grand jury.

Mueller was appointed on May 17, 2017, by Rosenstein. Rosenstein took oversight of the probe after Sessions recused himself due to his early work on the Trump campaign in 2016.

Mueller’s investigation, which was initially ordered to look into the 2016 election, has gone on for more than a year and a half. It has expanded to probe financial crimes of Trump associates before the election, conversations Trump’s national security adviser had with the Russians during the transition and whether Trump obstructed justice with his comments and actions related to the probe.

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Stone was charged with obstruction of justice, witness tampering, and making false statements to Congress after being indicted last month as part of Mueller’s probe.

Twenty-six Russian nationals and three Russian companies have also been charged with interfering in the 2016 presidential election. But none of the Trump associates have been charged with crimes related to collusion.

Other convictions include former national security adviser Michael Flynn and former campaign adviser George Papadopoulos, who both pleaded guilty to making false statements in 2017. Former campaign adviser Rick Gates in 2018 pleaded guilty and former campaign chairman Paul Manafort was convicted and later pleaded guilty in a separate financial crimes case dating back before the 2016 election.

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Former Trump lawyer Michael Cohen pleaded guilty to making false statements in a case brought by Mueller in November. Alex van der Zwaan, a London-based lawyer, pleaded guilty to making false statements this year, and Richard Pinedo, a California man, pleaded guilty to identity fraud in 2018.

The Associated Press contributed to this report.