In the more shadowy case, which involves an unknown person summoned before a grand jury this summer, the D.C. Circuit on Monday set a separate round of arguments for Dec. 14.

The case traveled in recent months from U.S. District Court Chief Judge Beryl Howell to the U.S. Court of Appeals for the D.C. Circuit, back down to Howell and back up again to the appeals court with most details shrouded in secrecy, another indication that much of Mueller’s activity is taking place behind the scenes and is rarely glimpsed by the press or public.

Nothing on the two dockets for the mystery grand jury fight mentions Mueller or his team.

However, a POLITICO reporter who visited the appeals court clerk’s office on a day when a key filing in the dispute was due earlier this month observed a man request a copy of the special counsel’s latest sealed filing so that the man’s law firm could craft its response. The individual who asked for the secret filing declined to identify himself or his client and replied “I’m OK” when offered a reporter’s business card to remain in touch.

Three hours later, a sealed response in the grand-jury dispute was submitted to the D.C. Circuit.

Another detail emerged Wednesday strengthening the secret legal battle’s apparent tie to Mueller’s probe.

The first appeal appears to have been rejected by a D.C. Circuit panel as premature. The witness’s lawyers asked the full bench of the appeals court to review that decision but a notation in court files says only nine of the court’s 10 active judges participated. Bowing out was Judge Greg Katsas, the court’s only member appointed by President Donald Trump.

Katsas served as a deputy White House counsel before Trump tapped him for the powerful D.C. Circuit last year. At Katsas’s confirmation hearing, he acknowledged working on some issues related to the Russia investigation and signaled he would take a broad view of his recusal obligations stemming from that work.

“In cases of doubt, I would probably err on the side of recusal,” Katsas told senators last October.

A spokesman for Mueller’s office, Peter Carr, declined to comment on the litigation.

Lawyers for Trump said the mystery grand jury subpoena fight moving through the D.C. Circuit had nothing to do with the president. “No idea,” said Trump personal attorney Jay Sekulow when shown the docket for the case.

Attorneys for Stone said they didn’t know of anyone challenging a Mueller subpoena beyond Miller. Miller’s lawyers agreed with Mueller’s prosecutors to make many aspects of that dispute public.

No such agreement appears to have been struck in the other fight, although Mueller’s team and the mystery witness did file a joint motion earlier this month asking the appeals court to expedite resolution of the dispute.

Several other lawyers who represent witnesses in the Mueller investigation also said they were unaware of who’s crossing legal swords with the special counsel’s team in the largely secret case.

A few, bare-bones details about the dispute are available in the public record. While all records about the litigation in the district court are sealed, the D.C. Circuit’s docket shows that the case was brought in the District Court on Aug. 16 and Howell ruled on it on Sept. 19. The initial appeal was filed five days later.

“The bottom line is the most likely scenario is someone filed a motion to quash or otherwise resisted a grand jury subpoena, and the judge issued an order denying that and saying the witness needs to testify,” said Ted Boutrous, a Gibson Dunn & Crutcher attorney who has handled grand jury-related litigation for journalists and media organizations.

It’s unclear whether the case the appeals court has agreed to hear in December involves an assertion of attorney-client privilege or some other privilege, is framed as a broader attack on Mueller’s authority, or perhaps advances both sets of arguments.

“It’s very hard to tell from this docket,” Boutrous said.

The grand jury cases pose a threat to Mueller’s investigation because they can serve as vehicles to get questions of his authority and legal legitimacy before appellate judges relatively quickly. Such questions have also been raised by defendants in some of Mueller’s criminal cases, but all the human defendants who have set foot in a courtroom have ultimately decided to plead guilty and drop any challenges to the special counsel’s authority or tactics.

One Russian company charged in and fighting a Mueller case, Concord Management and Consulting, has attacked the special counsel’s authority, but the judge turned down the motion. Concord attempted an immediate appeal, but since dropped it.

Mystery cases moving through the courts involving grand jury matters and independent counsel matters aren’t uncommon. A 1997 conflict-of-interest investigation into AmeriCorps chief Eli Segal’s fundraising activities was conducted under seal from its start. And a final report remains out of public view involving another Clinton-era probe into Labor Secretary Alexis Herman and influence peddling accusations.

“This can get a step or two weirder than it already is,” said an attorney representing a senior Trump staffer in the Russia inquiry. The lawyer recalled a case from a previous investigation where he couldn’t even get to see a judge’s opinion because it was under seal.

“It could be anyone who’s been subpoenaed by the special counsel for anything,” the attorney added.