A federal appeals court announced Friday that it would hold a closed oral argument session on Dec. 14, to address a lawsuit apparently filed against Special Counsel Robert Mueller by an unidentified grand jury witness.

Much of the detail in the case remains under seal, and there has been intense speculation in the legal and political communities over the identity of the mystery appellant.

Speculation in some media was that President Trump himself had filed the lawsuit back in August, but his attorney denied it.

The suit could be related to a possible subpoena for the witness to appear before the federal grand jury empaneled by Mueller, who is looking into a range of issues, including the extent of Russia interference in the 2016 election.

This case is not related to the appeal by Andrew Miller, a former associate of Trump confidant Roger Stone, who is resisting a Mueller subpoena. Miller’s case was argued Nov. 8 before the same D.C.-based appeals court.

The three-judge panel hearing the mystery case includes those appointed by Presidents Bill Clinton, George W. Bush and Ronald Reagan.