The report by The New York Times that Michael Flynn has withdrawn from a joint defense agreement with President Donald Trump might indicate that he is cooperating with special counsel Robert S. Mueller III. If so, this could be a significant turning point in the investigation.

First, what is a joint defense agreement? A joint defense agreement is a pact among attorneys for multiple targets or subjects in a criminal case in which they agree to share information. The agreement may be written or unwritten. Any joint defense agreement will be defined by its explicit terms, but generally, under such an agreement, attorneys have a duty to keep the confidences of all of the clients covered by the agreement. The attorneys also have a duty to avoid conflicts of interest as to any of the clients. The attorneys can compare notes, allocate work efficiently by dividing tasks and avoiding duplication, and develop a unified strategy.

The main advantage of joint defense agreements is that the information that they share is protected by a form of the attorney-client privilege, known by some courts as a joint interest privilege. These agreements can help targets or subjects sidestep the so-called prisoner’s dilemma, in which they must decide in a vacuum whether to help each other by remaining silent or betray each other by cooperating with authorities. When subjects or targets form a unified defense strategy, it is more difficult for prosecutors to “flip” targets, and use them as cooperators against their co-conspirators.

In the special counsel’s investigation, it has been reported that members of the administration have entered into a joint defense agreement. This makes sense because as they field requests from Mueller and his team for documents and interviews, they can work together to share the work and develop a unified defense strategy.

But what does it mean if Flynn has decided to withdraw from the defense agreement? It could mean that he and his attorney have decided that his interests have diverged from the other members of the agreement. Perhaps Flynn and his attorney have decided to pursue a different strategy. For example, they may decide against voluntarily turning over documents and instead to litigate disclosure issues in court. But such details can usually be worked out within the defense team. For that reason, it seems more likely that Flynn has withdrawn from the agreement because he has decided to cooperate with Mueller to provide truthful information and possibly testimony in exchange for leniency for any crimes of which he is convicted.

Recent reports suggest that Flynn has significant exposure to criminal prosecution. Mueller effectively fired a shot against Flynn’s bow when he charged Paul Manafort with violating the Foreign Agent Registration Act, among other offenses. Similarly, reports say that Flynn belatedly filed notice with the Department of Justice regarding his own lobbying work for the government of Turkey. Even more concerning, other reports indicate that Flynn participated in meetings to discuss the kidnapping and rendition to Turkey of cleric Fethullah Gulen from his refuge in Pennsylvania. Gulen is a rival of Turkish President Recep Tayyip Erdoğan.

The congressional testimony of former acting Attorney General Sally Yates in May provides further evidence of criminal exposure for Flynn. Her testimony about Flynn’s contacts with Russians suggest that his conversations may have been intercepted by a wiretap authorized by the Foreign Intelligence Surveillance Court. This surveillance could yield a great deal of incriminating information about Flynn and his contacts with Russia on behalf of the Trump team. Yates was careful to protect classified information during her testimony, so we do not know all of the details about his conversations, but Yates testified that she was concerned that Flynn was “compromised.” If so, he could be charged with acting as an agent of a foreign government while serving as a U.S. government official.

All of these potential charges provide an incentive for Flynn to cooperate against others. And even if Flynn is unwilling to cooperate to obtain leniency for himself, he may be willing to do so for his son, Michael Flynn Jr. Reports indicate that Flynn Jr. also faces potential charges arising from his work as chief of staff for his father at Flynn Intel Group. Prosecutors will sometimes agree to give what is known as “third-party credit” in the form of leniency to another person in exchange for a defendant’s cooperation.

If, in fact, Flynn is cooperating, this development could be very significant for Mueller’s investigation. As a member of the foreign policy team on Trump’s presidential campaign, he likely has information about any contacts with the Russian government by members of the campaign. Flynn may be able to provide the crucial links between all of the disparate pieces of evidence that have come to light to date—the June 2016 meeting with Russians to obtain disparaging information about Hillary Clinton, the overtures for meetings with George Papadopoulos, the travels of Carter Page. A cooperator is often disliked by a jury because of his own crimes, but if his testimony can be corroborated by other independent evidence, such as telephone records, bank records or surveillance recordings, then he can become a powerful narrator who can pull together the pieces of the story in a way that makes sense.

And if there was substantial cooperation between the Trump campaign and the government of Russia to influence the election, then Trump’s request to former FBI Director James Comey to “letting Flynn go” takes on a more ominous tone, and becomes a more egregious alleged case of obstruction of justice. Ironically, Flynn’s cooperation against Trump could provide the basis for charges against Trump for his efforts to protect Flynn.