Washington is abuzz over a mystery case involving the special counsel Robert Mueller, an unnamed witness, and a grand-jury subpoena.

The case has been shrouded in secrecy since it began in August, and few details have trickled out.

This week, we learned that the unidentified witness in the case is not a person but a corporation from a foreign country.

On Tuesday, the US Court of Appeals for Washington, DC, handed prosecutors a victory when it determined that the unnamed corporation has to comply with the grand-jury subpoena.

A Justice Department veteran who's been following the case said that "at the end of the day, these items will become known to a grand jury" and potentially the American public.

Fervent observers of the special counsel Robert Mueller's investigation into Russia's interference in the 2016 US election have been captivated over the past few months by a protracted court battle between Mueller and an unknown party over a grand-jury subpoena.

The case has been shrouded in secrecy since it began in August, and few details have trickled out.

Mueller's office appears to have mounted the case, referred to in court documents as "Sealed v. Sealed," on August 16.

CNN spotted several lawyers from Mueller's office entering a courtroom at a federal courthouse in Washington, DC, in September.

They were arguing against an unknown defense team before Chief District Judge Beryl Howell, who oversees cases related to federal grand juries.

Politico also confirmed a link between the case and Mueller's team when a reporter working for the outlet overheard a man request a document in the case from Mueller's office. Politico reported that the man declined to identify himself when approached by the reporter.

On September 19, Howell issued a ruling.

Five days later, one of the parties in the case appealed Howell's ruling to the US Court of Appeals for the DC Circuit.

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Nelson Cunningham, a former federal prosecutor and Senate and White House aide, wrote in Politico that the quick handling of the case and the appeal was noteworthy.

Not only did the unidentified witness appeal the lower court's ruling almost immediately, but the appeals court responded quickly. The court set an accelerated briefing schedule, and all the judges involved moved quickly to amend a procedural flaw in just under a week, something that typically would have taken "weeks or months to resolve," Cunningham wrote.

The unnamed witness in the case also took the unusual step of requesting a hearing before all 10 of the DC Circuit Court judges. One of them, Gregory Katsas, recused himself from the matter. Katsas was appointed to the court by President Donald Trump and previously worked in the Office of White House Counsel as deputy counsel to the president.

U.S President Donald Trump and U.S. first lady Melania Trump stand at the North Portico before a State Dinner with French President Emmanuel Macron and French first lady Brigitte Macron at the White House, April 24, 2018 in Washington, DC. Getty Images/Chris Kleponis

An entire floor of a federal courthouse went into lockdown mode

The circuit court initially declined to hear the case, sending it back to Howell, who oversaw another sealed hearing on October 5. CNN reported that Michael Dreeben, a veteran attorney working for Mueller's office who specializes in trial law, was in the room during the arguments.

The appellate court eventually decided to take up the case and set a date for December 14 for arguments before a three-judge panel:

On that day, as reporters milled about the room on the fifth floor of the courthouse where arguments were due to take place, building security put the floor on lockdown.

CNN reported that security officers cleared the entire floor, including the courtroom — except for a few law clerks who assist judges — the hallway, the vestibule, the stairwells, and the elevator banks.

Officers reportedly also checked a coat closet on the floor to make sure nothing was left behind. At one point, CNN said, the elevator didn't even open onto the fifth floor.

Over an hour passed with no sign of any attorneys entering or leaving the courtroom or the building. Then, about 10 minutes after activity at the courthouse appeared to wrap up for the day, reporters spotted a black car with attorneys on Mueller's team arriving back at the special counsel's office.

The unknown witness isn't a person — it's a foreign company

Robert Mueller. AP Photo/J. Scott Applewhite

The mystery case took another interesting turn on Tuesday when the appeals court issued a ruling revealing that the unnamed witness in the case isn't a person but a foreign corporation, described in documents only as being "owned by Country A."

The ruling indicates that the company has been fighting a subpoena from Mueller's office to hand over information to the grand jury, arguing that doing so would violate the law in "Country A."

The document said the court rejected the company's claim that complying with the subpoena would do so and ordered it to hand over the information. It also revealed that the corporation is being fined every day that it doesn't comply with the subpoena.

Read more: The 20 biggest Trump-Russia bombshells of 2018

Jeffrey Cramer, a longtime former federal prosecutor who spent 12 years at the Justice Department, told INSIDER that the drip-drip nature of the case raised "many more questions" than answers.

A few things, however, can be gleaned from it so far.

First, the foreign company has materials Mueller wants — those could be in the form of emails or documents. The company also most likely has a US presence, Cramer said, and the ability to garner support in "Country A."

"Prosecutors normally gain documents from a foreign entity via a Mutual Legal Assistance Treaty request," he added. "That could have been requested here and resulted in a court fight. If there was no US presence, the foreign country could have simply denied the request."

Moreover, it's unusual that a government regulator from "Country A" supported the corporation's legal argument, as indicated in the appellate court's ruling.

Regardless, the ruling indicates that the company will now have to comply with the subpoena.

"At the end of the day, these items will become known to a grand jury," and potentially the American public, depending on how they're used, Cramer said.