With about 300 words in a Monday evening court filing, special counsel Robert Mueller revealed a major twist in his ongoing Russia investigation: that Paul Manafort — the former Trump campaign chairman who, after his conviction, agreed to cooperate with prosecutors — had violated his plea deal by repeatedly lying to investigators since signing it.

Mueller did not say much else — and Manafort’s own attorneys disagreed, claiming in the filing that Manafort “believes he has provided truthful information” — but former federal prosecutors say the filing was both highly unusual and a sign that Manafort is in deep trouble.

We don’t know what Manafort allegedly lied about, what evidence Mueller has that he lied, and whether and how Mueller will publicly reveal such information. But former federal prosecutors tell TPM that it is likely the lies were about something major, that the evidence that Mueller has that Manafort lied is convincing and that it will be Mueller’s call whether he wants to put such information into the public domain.

“It’s not taken lightly,” defense attorney and former federal prosecutor Patrick Cotter said of Mueller’s move to blow up his plea agreement with Manafort. “Whatever he lied about, it must be significant.”

Manafort’s violation of his plea deal puts him in very serious jeopardy. He’s already been convicted of eight counts in Virginia, where sentencing in the bank fraud and tax fraud case is scheduled for February, and as part of his plea in DC admitted to two more crimes: obstruction of justice, in the form of witness tampering, and conspiracy against the U.S.

“These cooperation agreements are written in such a way that you’re much worse off if you do this,”Alex Whiting, a former federal prosecutor said, arguing that Manafort would have been better off had he had just pleaded guilty without agreeing to cooperate.

“You’ve committed additional crimes that could enhance your penalty,” Whiting, who is now a Harvard Law professor, told TPM.

Mueller claimed in Monday’s court filing that Manafort “committed federal crimes by lying to the Federal Bureau of Investigation and the Special Counsel’s Office on a variety of subject matters.” According to Whiting, prosecutors are typically prepared for some reluctance on the part of cooperators to be completely forthcoming and are willing to confront them to “give them some chances to get back on track.”

“I’m sure that happened here,” Whiting said. “The fact that he continued to lie, to me indicates — and the judge is likely to see it this way — is that this was all a game … this was never being done in good faith.”

Furthermore, Mueller wouldn’t claim to a court that Manafort was lying if it was based on a “hunch,” said Nick Akerman, a former Watergate prosecutor.

“Mueller would never go to the mat on this unless he has got slam dunk proof that Manafort is lying” Akerman told TPM. “This is too high profile of a case to bluff. You would absolutely, only do this if you were certain he was lying.”

What happens next is less clear to former prosecutors, who said that for a cooperator to breach his or her plea agreement is extremely rare, given how exposed doing so would leave them.

The special counsel promised the judge, U.S. District Judge Amy Berman Jackson, who is presiding over the Manafort case in D.C., that it he would provide more details setting “forth the nature of the defendant’s crimes and lies” ahead of Manafort’s sentencing.

Mueller could seek to put some or all of those filings under seal, and cite the sensitivity of the ongoing investigation as reason for doing so.

“If Mueller wants to have the hearing in public, he can put on all the evidence [that Manafort lied] and make broadly public to the world,” Paul Rosenzweig, who worked for Ken Starr’s Whitewater investigation, said. “It could be he is going to use this as a way of putting more information on the public record.”

While losing Manafort as a witness is no doubt a blow to Mueller’s investigation, it’s not without its silver linings, Cotter said.

“It’s an opportunity for Mueller to really send a message to all of his cooperating witnesses, now and in the future: do not screw with me.”