A mysterious case playing out in Washington amid tight secrecy – and presumed to involve the special counsel Robert Mueller – has been revealed to concern an unnamed corporation, owned by an equally anonymous foreign country.

The US circuit court of appeals on Tuesday issued a ruling that answered some of the questions in a judicial drama that has increasingly obsessed Mueller-watchers intrigued by the exceptional lengths to which the US government has gone to keep it secret. In several other regards, however, the judgment merely deepened the mystery.

The case, referred to in public dockets as 18-3071 with the evocative title Sealed v Sealed, began in August. All that was then known was that it related to a dispute between a grand jury and an unnamed party against whom the grand jury had issued a subpoena.

With all documents in the proceedings closed to public scrutiny, and with Politico reporting in October that one of its journalists had overheard someone in the appeals court clerk’s office requesting Mueller’s latest sealed filing in the case, a mushroom cloud of speculation was unleashed.

Leading observers of the Mueller inquiry speculated that the party resisting the grand jury’s subpoena could be none other than Donald Trump, though that idea was later debunked. Others wondered about the vice-president, Mike Pence, being at the center of it all.

The frenzy reached fever pitch last Friday when several reporters were physically cleared from the entire fifth storey of the DC circuit court before a closed-door hearing. Lawyers from both sides were secreted into the courtroom undetected and undercover.

Tuesday’s ruling dispenses with some of the most lurid theories concerning the case. We now know that the grand jury’s subpoena refers not to an individual but to an anonymous corporation “owned by Country A”. The corporation had been trying to resist handing over information to the grand jury on grounds that to do so would be a breach of the law in the anonymous sovereign country that owns it.

The ruling also reveals that a federal district court in DC had imposed a fine on the corporation for every day it failed to meet the terms of the subpoena. The appeals court affirmed that fine, and insisted that the corporation had to hand over to the US government the information demanded in the subpoena.

Despite such glimpses of light, the case remains shrouded in darkness. What the grand jury wanted to find out is still unknown, as is the identity of the corporation or the country in which it is based.

The idea that the subpoena concerned Donald Trump was later debunked. Photograph: UPI/Barcroft Images

Nor is Mueller’s name mentioned anywhere in the new judgment. The absence of any reference to the special counsel means that his involvement in the case is still a distinct possibility, though not certain.

The US government clearly considers the case sensitive enough to pursue it amid almost unheard of furtiveness. Sealed v Sealed is also of sufficient importance for the justice department to merit proceedings being conducted at breakneck speed.

The case has passed from the DC district court to the appeals court twice in less than four months, astounding legal experts.

“That’s lightning fast. You would normally expect that number of different actions to take 12 months minimum,” said Bradley Moss, a prominent national security attorney.

Moss said that the involvement of a foreign-owned corporation in such a supremely sensitive and high-profile case suggested a bank with possible financial dealings with Trump or his family business. “This suggests to me that Mueller is following the money – Trump is heavily dependent on foreign banks as US lenders gave up on him a long time ago.”

Last month Deutsche Bank, Germany’s biggest lender, which is also the biggest lender to Trump, having provided almost $400m in loans, had its Frankfurt offices raided as part of an investigation into alleged money laundering. Trump has also had well-documented financial dealings with Saudi Arabia and Russia, where according to his former legal fixer Michael Cohen he tried to develop a Moscow tower through much of the presidential election year 2016.

Given the secrecy surrounding the case, though, there is no indication that any of these countries or corporations were involved in any way in case 18-3071.