A federal appeals court on Tuesday shot down a challenge mounted by an associate of veteran political operative Roger Stone, who argued the appointment of Robert Mueller as special counsel was unconstitutional.

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

In the legal fight, Miller argued that Mueller hadn’t been properly appointed, making his grand jury subpoena invalid. A lower court held Miller in contempt for refusing to testify. Miller’s attorney Paul Kamenar has said the issue of Mueller’s appointment is bound to be decided by the Supreme Court.

The judges – Karen LeCraft Henderson, Judith Rogers and Sri Srinivasan – ruled Mueller’s appointment is valid.

“Special Counsel Mueller was properly appointed by a head of Department, who at the time was the Acting Attorney General,” the 16-page opinion authored by Rogers said.

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

Stone was arrested last month and has pleaded not guilty to charges he lied to Congress, engaged in witness tampering and obstruction. The charges stem from conversations he had during the campaign about WikiLeaks, the anti-secrecy group that released material stolen from Democratic groups, including Hillary Clinton’s campaign.

The development comes as speculation regarding the Mueller probe report’s release date has reached a feverish pitch. The Justice Department told NBC News, which was one of several news outlets to report that the special counsel’s findings would be released as early as next week, said Attorney General William Barr will not receive the final report in the immediate future.

Meanwhile, a legal fight over what details in the report will be released has begun brewing. Appearing on ABC’s This Week Sunday, House Intelligence Committee chairman Rep. Adam Schiff (D-CA) vowed to subpoena the report to make both it and the “underlying evidence” public.

“If you take the position that the president cannot be indicted and the only remedy for improper, illegal or other conduct is impeachment, then you cannot withhold that information from Congress,” Schiff told anchor George Stephanopoulos. “That cannot be allowed to be the case. Bill Barr has committed in his testimony to making as much of the report public as he can, and the regulations will make it all. And we insist on it becoming public.”

On Monday, Deputy Attorney General Rod Rosenstein tampered down expectations that details of the report could be made available to the public, saying that it is uncommon for federal prosecutors to reveal aspects of an investigation outside of the scope of criminal charges. “The guidance I always gave my prosecutors and the agents that I worked with during my tenure on the front lines of law enforcement was: If we aren’t prepared to prove our case beyond a reasonable doubt in court, then we have no business making allegations against American citizens,” Rosenstein said in a speech before the Center for Strategic and International Studies.

This case is In re Grand Jury Investigation, No. 18-3052 in the U.S. Court of Appeals for the District of Columbia Circuit.

The Associated Press contributed to this report.