WASHINGTON — A mystery witness fighting a secret fight over a sealed subpoena with a prosecutor who may be Robert S. Mueller III, the special counsel investigating the Trump-Russia affair, turns out to be a corporation owned by a foreign government, according to a federal appeals court ruling on Tuesday.

The three-page ruling by a panel of the United States Court of Appeals for the District of Columbia Circuit described the unnamed witness in those terms, but it did not identify it any further, nor did it confirm whether the prosecutor is the special counsel, as has been widely speculated.

During closed-door oral arguments before the panel last week, court security officers took the unusual step not only of closing the courtroom, but also of clearing the entire floor of the courthouse so that reporters could not see who the lawyers were as they went in and exited.

[Read the judgment.]

But the new details about the witness’s identity offered some tantalizing clues to a mystery that has riveted Washington journalists and legal insiders who are closely watching Mr. Mueller’s investigation, eliminating some theories about whom it may be while opening the door to new a new round of guessing.