Former national security adviser Mike Flynn pleaded guilty to lying to F.B.I. investigators, becoming the fourth individual involved in the Trump campaign to face charges in the ongoing Justice Department investigation into Russia’s interference in the 2016 presidential election, and the second who appears to have cut a deal with special prosecutor Robert Mueller.

The charge against Flynn, according to a court filing made public Friday, is that he “did willfully and knowingly make materially false, fictitious, and fraudulent statements” during a January 24 interview in which he discussed his conversations with Russian Ambassador Sergey Kislyak. Prosecutors charge that the retired three-star general falsely stated that he did not ask Kislyak to “refrain from escalating the situation” in response the sanctions leveled against Moscow by Barack Obama, or request the ambassador to “delay the vote on or defeat” a pending United Nations resolution condemning Israeli settlements. (Flynn came under intense scrutiny for his communications with Kislyak earlier this year, ultimately resigning because of them.)

Flynn later released a statement addressing his plea, saying he “accept[s] full responsibility” for his actions, and that his decision to plead guilty and to cooperate with Mueller’s team was, in his eyes, “in the best interests of my family and of our country,” but holding that any “accusations of ‘treason’ and other outrageous acts” are false. Trump’s lawyers, meanwhile, continued their campaign to soothe the president, with Ty Cobb releasing a statement calling Flynn “a former Obama administration official“ and emphasizing the single charge against him. “Nothing about the guilty plea or the charge implicates anyone other than Mr. Flynn,” the statement read. “The conclusion of this phase of the Special Counsel's work demonstrates again that the Special Counsel is moving with all deliberate speed and clears the way for a prompt and reasonable conclusion.”

The fact that Flynn was not charged with any other crimes, as well as the fact that Mueller did not convene a grand jury to secure an indictment, immediately fueled speculation that Trump’s former top adviser is cooperating with the government. For months, a steady drip of news reports have suggested a web of clandestine lobbying efforts by Flynn and his son Michael G. Flynn on behalf of foreign governments including Turkey. Mueller’s apparent hesitance in charging Flynn, despite reports that he already had enough evidence to arrest the pair, was widely interpreted to mean that Flynn’s lawyers were working on a deal. Several days ago, Trump’s lawyers confirmed that the former adviser’s attorney stopped sharing pertinent information tied to the probe with the White House legal team—a move that seemed to confirm that Flynn had, in fact, flipped.

“On its face, it is a really, really good deal for him,” Sol Wisenberg, a white-collar criminal defense attorney who worked on the Whitewater investigation, told me—assuming that Flynn doesn’t face any additional charges and that immunity for his son is part of the agreement. “He is obviously cooperating. There is no way he is not cooperating, there is just no way,” Wisenberg told me. “If he wasn’t cooperating he would be indicted and I guarantee you there would be more counts than this.”

That conclusion was confirmed later Friday morning by ABC News, which reported that Flynn, facing mounting legal costs and feeling “abandoned” by Trump, had made the decision to cooperate with Mueller. At his sentencing, the judge reportedly said that Flynn, who faces up to five years in prison and a $250,000 fine, could see his sentence reduced “if and only if he provides substantial assistance in the prosecution of another person.”

In theory, Flynn’s testimony could prove damning to the Trump administration—particularly to Jared Kushner, who gave Flynn the initial instruction to contact foreign officials ahead of the U.N. vote, two former Trump transition officials told Bloomberg on Friday. But it could also deal a more glancing blow. If Trump or any of his campaign officials asked Flynn to get in touch with the Kremlin to discuss election meddling, whether directly or indirectly, “that’s dire,” noted journalist David French, but “highly unlikely.” If, on the other hand, Flynn was simply instructed to perform outreach, “we could be looking at less of a criminal conspiracy and more of a festival of lies surrounding a non-conspiracy.”

This post has been updated.