A mystery witness who is attempting to fight a grand jury subpoena from special counsel Robert Mueller's probe will get a closed-door hearing next month.

Court filings on Friday revealed that lawyers for the unidentified witness will make oral arguments before the DC Circuit Court of Appeals on December 14 in a secret closed-door hearing.

The identity of the mystery witness has baffled Beltway observers since the existence of the case was revealed last month. Some initially speculated that it could even be President Donald Trump, whose lawyers have previously vowed to fight any subpoena from Mueller.

But Trump's lawyers deny that he is party to the case, and legal experts say it is extremely unlikely that he is the witness.

A mystery witness who is attempting to fight a grand jury subpoena from special counsel Robert Mueller's (above) probe will get a closed-door hearing next month

Still, the limited information available in the court docket highlights some highly unusual aspects of the case.

Since it was filed in August, the sealed case has moved with extreme speed back and forth between the DC District Court and Circuit Court of Appeals.

'At every level, this matter has commanded the immediate and close attention of the judges involved—suggesting that no ordinary witness and no ordinary issue is involved,' former federal prosecutor Nelson W. Cunningham wrote in an op-ed for Politico.

Cunningham notes that when the witness lost the first appeal in Circuit Court, he (or she) immediately petitioned for rehearing en banc, the 'very unusual action' of convening all 10 judges on the Circuit to review the order.

On the order disposing of that petition, the sole recusal was Judge Gregory Katsas - Trump's only appointee to the Circuit.

However, Katsas' recusal doesn't necessarily mean that the witness is Trump or someone close to him, as Katsas has previously said he would recuse himself from issues related to the Mueller probe due to prior work he had done on Russia issues.

The case's link to Mueller was only publicly revealed in late October, when a Politico reporter staking out the appeals court clerk's office overheard a man asking for 'the special counsel's latest sealed filing' so his firm could craft a response. The man refused to identify himself or his client.

Then, on November 14, the mystery witness filed a sealed 6,487-word brief with the Circuit Court.

As speculation about the case has mounted, many have tried to make the case that Trump is the mystery witness.

However, his attorney Jay Sekulow has denied it. Others have asserted that Trump and his other attorney, Rudy Giuliani, would be unlikely to refrain from talking publicly about the legal battle if they were involved.

Experts say that in some sealed cases, even the decision is kept secret. If that happened with this case, the world may never discover the mystery witness's identity.