It's very likely that President Trump's former national security adviser and close confidant is now a confidant of an investigation into whether the Trump campaign coordinated with the Russians to meddle in the election.

There's one reason — one criminal charge, actually — that lets us safely assume Michael Flynn switched sides: He pleaded guilty to one charge of lying to the FBI, when there's so much more the special counsel could have potentially nabbed him for.

"[It] suggests a bombshell of a deal with prosecutors,” said Jens Ohlin, vice dean of law at Cornell, in an email. “The best explanation for why Special Counsel Robert S. Mueller III would agree to it is that Flynn has something very valuable to offer in exchange: damaging testimony on someone else.”

“One doesn't plead guilty unless you hope to get something out of it,” said Jack Sharman, who was special counsel to Congress during the Whitewater investigations into Bill and Hillary Clinton's real estate affairs.

It's easy to see how Flynn benefited: He could have been hit with much more serious charges, and they might have gone after his family. The special counsel had convened a grand jury to look at a range of things Flynn had allegedly done wrong. His failure to register his work lobbying for foreign governments is one of the big ones. The special counsel was even looking into Flynn's son to see whether they could pressure Flynn to talk.

"I recognize that the actions I acknowledged in court today were wrong, and, through my faith in God, I am working to set things right. My guilty plea and agreement to cooperate with the Special Counsel’s Office reflect a decision I made in the best interests of my family and of our country. I accept full responsibility for my actions,” Flynn said in statement.

The independent investigation looking into whether the Trump campaign helped Russia meddle in the 2016 presidential election seems to have a pretty good idea about how Flynn could be helpful, even if publicly, we don't. The Post's national security team reports that Flynn's lawyers met with Mueller's team before this deal and broke down what information Flynn could provide. After that, Flynn started meeting with Mueller's team. After that, Flynn was charged with just one crime.

Trump's former national security adviser Michael Flynn pleaded guilty on Dec. 1 to lying to the FBI about his contacts with Russian Ambassador Sergey Kislyak. (Jenny Starrs/The Washington Post)

Contrast that with Mueller's aggressive tactics in other aspects of this investigation. He has signaled he's willing to throw the book at people who don't cooperate. He showed up at the home of Trump's former campaign chairman, Paul Manafort, in a predawn raid this summer, and he's charged Manafort with money-laundering and fraud. Manafort recently put up four homes worth $11.6 million as collateral to the government just so he could get out from under house arrest.

By contrast, Flynn pleaded guilty to the same crime that a much lower-level campaign aide, George Papadopoulos, pleaded guilty to: Lying to investigators.

Legal experts said Mueller has all the leverage in this deal with Flynn. It's common in a plea deal like this for prosecutors to use the threat of more charges if the defendant doesn't cooperate, said Jeffrey Jacobovitz, a white collar lawyer.

'Lock her up': How Michael Flynn talks about law enforcement (Joyce Koh/The Washington Post)

And Sharman said cooperation is entirely in the eye of Mueller. He says the conversation with Flynn and Mueller probably went something like this:

“You'll cooperate, you'll tell us the truth. If you do all that, then at the end of the day, we'll say nice things about you to the judge, who is to impose a sentence on you. If you don't do those things, we won't be nice. And by the way, if you don't take this deal, let me tell you what we will charge you with in the grand jury.”

So it's no question that Flynn is talking with prosecutors. If he doesn't, he could face even more jail time.  The question haunting the White House now is: What does Flynn know that he's about to share?