For months now, close watchers of the Mueller investigation have been intrigued by a mysterious court appeal involving a challenge to a grand jury action that appears to be tied to the probe — though most of the details remain tantalizingly sealed.

The case is about a corporation owned by a foreign country. The Washington Post reported Tuesday that the corporation in question is a “foreign financial institution” subpoenaed in connection with the Mueller investigation. The company has sued to try and block that subpoena.

So far, things haven’t gone well for the company in court. A district judge ruled against them, found them in contempt, and imposed a $50,000 per day financial penalty. A circuit court panel ruled against them too.

Now the matter has reached the Supreme Court. On Tuesday, the court denied the company’s request to put that financial penalty on hold pending an expected appeal. Also this week, the company asked the court for permission to submit a sealed appeal that will keep their identity hidden. If the company does appeal, the court will then have to decide whether to take up the case.

The bigger picture, though, is that we still don’t yet know which corporation or which country all this is about. And though there’s a lot of circumstantial evidence that Mueller is involved, the only confirmation of that so far has come from anonymous sources. So much of the intriguing mystery still remains.

Why do people think this appeal even has anything to do with Mueller?

For many months, no one managed to confirm, either on the record or from sources, that the appeal even involves the special counsel at all. The details were very closely held.

That changed on Tuesday, when the Washington Post’s Robert Barnes, Devlin Barrett, and Carol Leonnig cited two sources familiar with the case as claiming that it was in fact about the Mueller probe.

Even before that, we got a good deal of circumstantial evidence at this point that did appear to point to a Mueller tie:

On an October day when a filing in this particular case was due, a Politico reporter saw someone at the relevant courthouse ask for the special counsel’s latest sealed filing, so his law firm could work on the response. (Mueller is the only special counsel around.)

Politico’s Josh Gerstein and Darren Samuelsohn also observed that when the case reached the appellate level, Judge Gregory Katsas recused himself. Katsas was a staffer in Trump’s White House counsel’s office before Trump appointed him to the court, and he had previously said he’d recuse himself on Russia investigation-related matters.

Then, when appellate arguments were held in December, the courthouse sealed off the whole floor the courtroom was on, so reporters couldn’t see which lawyers would enter. But shortly after arguments appeared to end, CNN saw several Mueller prosecutors returning to their office building — including their top appellate specialist Michael Dreeben (the one person nearly certain to be involved if this was a Mueller-related appeal), as well as international specialist Zainab Ahmad.

Finally, when the appellate panel’s ruling was announced in December, the Washington Post saw five Mueller prosecutors (including Dreeben) attend a sealed hearing in district court, along with two other unidentified lawyers.

Additionally, there was a different witness subpoenaed by Mueller’s grand jury who has sued the government — Roger Stone associate Andrew Miller. His suit and appeal have been handled similarly to this one, though more of the details are public. (Appellate judges haven’t yet ruled on it.)

What do we know for sure about this mysterious appeal?

For a while, all we really knew was the timing of certain actions, from court dockets:

The suit was filed by the corporation in DC district court on August 16, 2018.

Judge Beryl Howell ruled in favor of the government on September 19.

The corporation appealed, but at first their appeal was dismissed, perhaps for procedural reasons. Judge Gregory Katsas recused at this point.

The corporation appealed again on October 10, and this time, the DC appellate court agreed to hear it.

Various sealed briefs and motions were filed over the next two months, leading up to oral argument in December.

Then the three-judge panel’s ruling on December 18 revealed more details. Most importantly, it made clear that the suit is about a grand jury’s subpoena for information from a certain corporation — a corporation owned by “Country A.”

“When the Corporation failed to produce the requested information, the court held the Corporation in contempt, imposing a fixed monetary penalty to increase each day the Corporation fails to comply,” the judges said.

On appeal, the corporation made three main arguments about why they shouldn’t have to comply with the subpoena:

First, they said they should be immune from the subpoena because of the Foreign Sovereign Immunities Act (FSIA), a law about how foreign countries can’t be sued in US courts.

Second, they argued that the DC federal district court lacked appropriate jurisdiction to consider the case.

Third, they said the laws of their own country — Country A — would block them from complying with the subpoena.

The judges rejected all three arguments.

They said the corporation should not be immune under FSIA because their conduct falls under its “commercial activities exemption.” The details are vague, but this somehow involves “an act” from the corporation tied to commercial activity that “had a direct effect” in the US, even though the act took place outside the US.

They said the district court indeed had appropriate jurisdiction here.

And they said they “were unconvinced that Country A’s law truly prohibits the Corporation from complying with the subpoena.”

So the judges ruled in favor of the US government.

On January 8, the same panel of judges released their full (albeit partially redacted) opinion in the case. This revealed a few more tidbits — that the subpoena was “served on a US office of a foreign corporation,” that the company has “considerable business” in the US, and that the company faces a financial penalty of $50,000 a day for not complying.

Then came the Washington Post report citing two sources to confirm that the company in question is a “financial institution,” and that the grand jury in question is in fact Robert Mueller’s.

What happens next isn’t yet clear, but a Supreme Court appeal could ensue.

What does it all mean?

If this is in fact Mueller’s grand jury we’re talking about, it certainly is intriguing that they want information from a corporation owned by a foreign country.

Considering what’s been reported about Mueller’s focus so far, there appear to be two particularly likely possibilities here:

A Russian company: For obvious reasons, this could be a state-owned Russian company. There have been claims that the bank VTB was involved in the Trump Tower Moscow talks, and the chair of a different bank, VEB, met with Jared Kushner during the transition. The Steele dossier made various uncorroborated claims about planned payoffs to Trump associates involving the oil company Rosneft. Trump transition adviser Erik Prince met the manager of a Russian sovereign wealth fund in Seychelles. (None of these companies have been accused of wrongdoing in connection with the Mueller probe.)

For obvious reasons, this could be a state-owned Russian company. There have been claims that the bank VTB was involved in the Trump Tower Moscow talks, and the chair of a different bank, VEB, met with Jared Kushner during the transition. The Steele dossier made various uncorroborated claims about planned payoffs to Trump associates involving the oil company Rosneft. Trump transition adviser Erik Prince met the manager of a Russian sovereign wealth fund in Seychelles. (None of these companies have been accused of wrongdoing in connection with the Mueller probe.) A Gulf state company: Many reports this year have made clear Mueller is scrutinizing money trails and influence operations from Gulf states — the United Arab Emirates, Saudi Arabia, and Qatar are said to have come under scrutiny. It’s unclear why exactly Mueller has focused on this, and no charges have been brought on the topic just yet.

Intriguingly, the Daily Beast’s Erin Banco recently reported that Mueller is soon expected to issue “court filings” on two intriguing topics: “Middle Eastern countries’ attempts to influence American politics,” and “Trump associates’ conversations about sanctions relief” for Russia. Perhaps this sealed appeal is related to either topic.

But of course, we should keep in mind that Mueller knows a lot we don’t know. It’s possible that the subpoena involves a totally unexpected country. Until we get more details, the mystery remains.