Special counsel Robert S. Mueller III has voided his cooperation agreement with former Trump campaign chairman Paul Manafort, accusing Manafort of violating it by lying.

The episode appears to be a setback for Mueller’s efforts to glean information from a key player in the Russia investigation. But it may double as an opportunity for disclosure — even a chance for Mueller to enter key events into the public record. That’s especially the case if he’s worried about what new acting attorney general Matthew G. Whitaker might do to the investigation and if the whole thing is indeed winding down. The episode could give us one of the best senses to date of exactly what Mueller is probing and how much trouble President Trump and his campaign might be in.

That is, if Mueller wants to spill.

In the filing voiding the agreement, Mueller’s team promises extensive information on Manafort’s alleged lies before he is sentenced: “The government will file a detailed sentencing submission to the Probation Department and the Court in advance of sentencing that sets forth the nature of the defendant’s crimes and lies.”

The words “detailed sentencing submission” loom large. Mueller’s team has always been tight-lipped publicly, but it has made some key disclosures through indictments (often referred to as “speaking indictments”). The indictments of 13 Russians earlier this year contained extensive detail and was perhaps more about disclosure than punishment, given the Russians almost definitely will never be in the United States to face trial. The initial Manafort indictment disclosed that he was viewed as a key witness not just generally but specifically with regards to potential Trump campaign collusion with Russia — a pregnant inclusion.

The Mueller filing on Manafort’s alleged lies won’t be an indictment, per se, and there is a very real chance it could remain under seal to protect the ongoing investigation. But if it is made public, it would give Mueller a chance to say whatever he feels like disclosing, and notably without Whitaker’s approval — which would be required for a new indictment or for releasing a final report, if Whitaker has indeed taken oversight of the probe and supersedes Deputy Attorney General Rod J. Rosenstein.

It’s tough to see how it wouldn’t shed at least some light on key events. Mueller has reached plea deals with others for lying to his team, but none were as central as Manafort, who ran Trump’s campaign for months. He was there for the Trump Tower meeting, for instance. He has long-standing ties to Russian interests.

There’s also the fact that Manafort previously reached the plea deal and ostensibly knew the punishment that would follow if he lied. The fact that he allegedly did so — enough to cause Mueller’s team to void the agreement — suggests the alleged lies may have had to do with substantial questions in the Russia investigation. And if Manafort lied to cover something up, it may become clearer what, in fact, is being covered up.

That’s a lot of what-ifs, and Mueller may continue to play things close to the vest and/or keep the document under seal. But rarely has there been so good an opportunity to learn just exactly where this investigation stands through court filings. And we seem to have entered a new phase in which external factors — mostly Whitaker — are threatening Mueller’s 18-month investigation as never before.

“[The filing] will set out not just alleged lies but explanations of why they are lies, which has to include the evidence Mueller has that contradicts Manafort‘s prevarications,” former Justice Department official Harry Litman said. “So it will be a rich document chock-full of information that we don’t know.

“But we (as opposed to the court) may not learn any of it yet, because it could be filed under seal to protect the ongoing investigation.”

Former federal prosecutor Patrick Cotter said expectations should indeed be tempered.

“My suspicion is that the Old Marine will continue to hold his fire,” Cotter said. “For guys like Mueller, not letting developments like the emergence of a legal nuisance like Whitaker rattle you is both habit and principle. My money says that Mueller will continue to steer a straight, steady and quiet course.”