WASHINGTON (AP) – A federal appeals court panel on Tuesday upheld Robert Mueller’s appointment as special counsel after his authority was challenged by a witness who had refused to appear before a grand jury related to the Russia investigation.

The case was brought by Andrew Miller, an aide to longtime Trump campaign adviser and confidant Roger Stone. Prosecutors wanted Miller to testify as part of Mueller’s investigation into Russian interference in the 2016 election and ties to President Donald Trump’s campaign.

Miller’s attorneys claimed Mueller’s appointment was unconstitutional, making the grand jury subpoena invalid. They argued that Deputy Attorney General Rod Rosenstein did not have the authority to appoint Mueller to lead the investigation. The court found that Rosenstein was authorized to appoint Mueller because he technically became head of the Justice Department — for the Russia probe — after then-Attorney General Jeff Sessions recused himself from overseeing the investigation. The opinion by a three-judge panel was written by Judge Judith Rogers, who was nominated to the court by President Bill Clinton.

Four separate federal judges have upheld Mueller’s appointment.