(CNN) A federal judge has rejected an attempt from an associate of Roger Stone to challenge a subpoena on the grounds that special counsel Robert Mueller's appointment was unconstitutional.
Andrew Miller, a former associate of longtime Donald Trump confidante Stone, was ordered to testify before the Mueller grand jury. Miller had challenged a subpoena for documents and testimony by arguing that Mueller's appointment was unconstitutional.
"Multiple statutes authorize the Special Counsel's appointment, and the official who appointed the Special Counsel had power to do so. For these reasons ... the witness's motion to quash the grand jury subpoenas is denied," US District Court for the District of Columbia Chief Judge Beryl Howell wrote in a 93-page opinion signed July 31.
Howell ordered the witness to appear for testimony "at the earliest date available to the grand jury, and to complete production of the subpoenaed records promptly."
Nearly two weeks ago, at least five prosecutors from Mueller's office and two attorneys representing Miller spent almost 90 minutes in a sealed court proceeding before Howell.
The courtroom activity was not open to the public, but Miller's attorneys previously told CNN they were challenging requests for Miller to provide information to Mueller's grand jury. The opinion does not use Miller's opinion by name.
Their public statements about a challenge to Mueller came before investigators indicted 12 Russian intelligence officers, and Stone surfaced as a person in contact with them during the campaign. Stone was not charged in the Russian hacking indictment last month.
"We're obviously disappointed with the court's decision," said Paul Kamenar, one of Miller's attorneys. "We are exploring our options to appeal this decision."
Miller's testimony would add to the list of Stone associates who have appeared before Mueller's grand jury, and it's the latest sign that Mueller's team is still investigating Stone.
Stone acknowledged that he was likely the unnamed US official in last month's indictment of 12 Russian intelligence officers. The indictment included messages exchanged with Guccifer 2.0, the identity US investigators say was used by Russian hackers, but Stone has argued that the messages he sent were "benign" and are not evidence "of collaboration or collusion."
Miller is not the first Stone associate to raise the prospect of challenging a subpoena from Mueller.
Earlier this year, former Trump campaign aide Sam Nunberg went on a cable TV blitz threatening to defy a subpoena from Mueller.
"Screw that," Nunberg told CNN the week he was ordered to appear. "Why do I have to go? Why? For what?"
But by the end of the week, Nunberg had reversed course and agreed to cooperate with the special counsel's investigation.