The US Court of Appeals for the District of Columbia Circuit ruled Tuesday that Special Counsel Robert Mueller’s appointment was lawful and constitutional, rejecting a challenge by Andrew Miller who was held in contempt for failing to comply with subpoenas Mueller served on him during the investigation into Russian interference in the 2016 presidential election.

The district court struck down Miller’s argument that Mueller’s appointment violated the Constitution’s Appointments Clause because:

(1) the Special Counsel is a principal officer who was not appointed by the President with the advice and consent of the Senate; (2) Congress did not “by law” authorize the Special Counsel’s appointment; and (3) the Special Counsel was not appointed by a “Head of Department” because the Attorney General’s recusal from the subject matter of the Special Counsel’s investigation did not make the Deputy Attorney General the Acting Attorney General.

The court rejected the first argument, saying Mueller was an inferior officer and that his appointment would have been unlawful if he were a principal officer: “Special Counsel Mueller effectively serves at the pleasure of an Executive Branch officer who was appointed with the advice and consent of the Senate.” As to the second argument, the court said that Congress has, in fact, by law authorized the Attorney General to delegate duties to inferior officers–i.e. Mueller. The court then quickly rejected the final argument, saying: “an Acting Attorney General becomes the head of the Department when acting in that capacity because an acting officer is vested with the same authority that could be exercised by the officer for whom he acts.”

The decision marks one of Mueller’s biggest wins in his legal tenure and further solidified his authority as Special Counsel.