Former FBI Director Robert Mueller, the special counsel probing Russian interference in the 2016 election, departs Capitol Hill following a closed door meeting in Washington, June 21, 2017. Credit: Andrew Harnik | AP

A federal appeals court in Washington unanimously affirmed the validity of special counsel Robert Mueller’s appointment Tuesday, rejecting a challenge brought by an associate of President Donald Trump’s embattled longtime adviser Roger Stone.

The ruling from the U.S. Court of Appeals for the D.C. Circuit comes after Stone pleaded not guilty to charges of lying to try to conceal his efforts to learn about the release of hacked 2016 Democratic Party emails.

The separate constitutional challenge to Mueller’s appointment was brought by Stone’s associate Andrew Miller, who has been trying to block a grand-jury subpoena from the special counsel’s office.

Miller was held in contempt by a lower-court judge after failing to appear before the grand jury as part of Mueller’s investigation into Russian interference in the 2016 presidential election.

It was not immediately clear whether the ruling from a three-judge panel would prompt Miller to agree to testify. Miller’s attorney, Paul Kamenar, said Tuesday he is disappointed with the decision and considering whether to ask the full D.C. Circuit to rehear the case or go directly to the Supreme Court.

A spokesman for Mueller declined to comment on the opinion Tuesday, or to say whether the special counsel would continue to seek Miller’s testimony before wrapping up its investigation.

Two district court judges in Washington — one nominated by a Democrat, the other by Trump — had already upheld the legitimacy of Mueller’s appointment in recent rulings. The opinion issued Tuesday was the first time an appeals court panel had reviewed the special counsel’s authority.

The judges — Karen LeCraft Henderson, Judith W. Rogers and Sri Srinivasan — rejected Miller’s argument that Mueller was named to the post unlawfully. The court said the appointment is valid in part because Mueller is supervised by the attorney general and “effectively serves at the pleasure of an Executive Branch officer” confirmed by the Senate.

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

Kamenar, Miller’s attorney, has said Stone’s indictment might make Miller’s testimony unnecessary.

“Since they’ve already indicted him, and since they wanted my client to testify about what he knows about Roger Stone’s connection with WikiLeaks and Julian Assange, it would seem to me that they may not need my client anymore before the grand jury,” Kamenar said, referring to the site that published the Democratic emails.

Kamenar acknowledged, though, that prosecutors might want his client’s testimony as they investigate others.

“He already told the FBI everything he knows in a voluntary, two-hour interview,” Kamenar said. “He was just a former aide to Roger, part time, handling administrative things for him, handling his website, and he did turn over a lot of documents that they asked for from his computer, etc., during the response to the subpoena, so they had all that information, which was essentially not much of anything.”

The case at the D.C. Circuit was argued one day after Trump ousted Jeff Sessions as attorney general. The Senate this month confirmed new Attorney General William Barr, who will now supervise Mueller.

At his confirmation hearing this month, Barr told lawmakers he “understands the need for independence and the importance” of protecting Mueller from interference. But he did not commit to releasing the final report generated by the special-counsel investigation.

In court, Miller’s lawyers argued that the special counsel was named in violation of the appointments clause of the Constitution and that Mueller has broad prosecutorial powers with little oversight.

The special counsel’s team told the court that the office has day-to-day independence but has to report major developments to Justice Department supervisors.

“It is not the case that the special counsel’s office is off wandering in a free-floating environment,” government attorney Michael Dreeben said in court.

Washington Post writers Spencer S. Hsu and Matt Zapotosky contributed to this report.