A mystery filing in the case of W. Sam Patten, the D.C. lobbyist who pleaded guilty to acting as an unregistered foreign agent in August, adds a curious wrinkle to what has generally been regarded as a relatively minor case.

On New Year’s Eve, prosecutors moved to keep a routine status update in Patten’s case entirely under wraps. The joint status reports filed after plea agreements but before sentencing have typically included a broad reference to the defendant’s ongoing cooperation.

The fact that Patten’s status report is entirely under seal could suggest that Patten, who has been seen as a more marginal figure in the foreign lobbying world, may in fact have more value to prosecutors, including special counsel Robert Mueller, than previously realized — or that a problem has developed with his cooperation.

In pleading guilty, Patten admitted that he helped a Ukrainian oligarch attend the Trump inauguration by illegally using a straw donor to purchase a $50,000 ticket to the event. The plea agreement required Patten to cooperate with the D.C. U.S. attorney’s office, Mueller’s office, and other law enforcement agencies.

Patten’s Ukrainian client — Serhiy Lovochkin — was the same politico who prosecutors said was Paul Manafort’s main paymaster during his time in Ukraine. Patten worked with alleged Russian spy Konstantin Kilimnik as well, who was charged with witness tampering in the separate Manafort case in June.

Prosecutors from the Mueller investigation attended Patten’s August plea hearing, signaling interest in the case.

D.C. federal prosecutors charged Patten with one count of lying to Congress and another count of failing to register as a foreign agent, after receiving a tip from the Senate Intelligence Committee. Patten’s case comes as investigations into the Trump inauguration heat up, amid allegations of pay-for-play that have swarmed the $107 million event.

Patten does not yet have a sentencing date. Prosecutors typically wait until they believe a cooperator has expended their usefulness before setting a sentencing date.