Michael Flynn pleaded guilty on Friday to lying to the FBI about his contact with Russia, in the biggest sign that Special Counsel Robert Mueller’s investigation is moving into a critical phase.

Flynn, Trump’s short-lived national security adviser, appears to have flipped on the Trump administration: He’s now cooperating with Mueller’s investigators. He pleaded guilty to one charge — lying to the FBI about his interactions with Kremlin officials — seemingly in exchange for Mueller’s dropping more serious charges. As the charging documents state, these are not “all the facts known,” just “sufficient facts” to prove the one offense to which Flynn agreed to plead guilty.

He’ll likely be testifying down the line about interactions between more senior Trump administration officials about their interactions with the Russians.

Here’s everything that you need to know about the whirlwind of new Russia news:

The charges are about two interactions that Flynn had with Russians that he later lied about to the FBI. The first is that he asked the Russian Ambassador to delay the vote on or defeat a pending United Nations Security Council resolution condemning Israeli settlements. The Russian government opposed the resolution.

Flynn lied to the F.B.I., too, about asking the Russian Ambassador not to escalate the diplomatic situation between the U.S. and Russia after the U.S. expelled Russian diplomats following the revelation that Russia meddled in the election. Russian President Vladimir Putin, with knowledge that the Trump administration would be softer on Russia, didn’t expel U.S. diplomats, a move Trump praised.

READ: Here’s what Mike Flynn lied to the FBI about

Flynn’s going to sing.

The charging documents, published Friday, indicate that Flynn was asked to reach out to the Russians by “a very senior member of the Presidential Transition Team.” Mueller’s going to follow this up the chain of command to high-ranking members of Trump’s entourage.

And all signs point to Mueller’s having enough dirt on Flynn to force him to comply with the investigation. Flynn isn’t cooperating with the investigation by choice, but because the charges against him could have been far more severe. And in order of him to testify that Trump officials had contact with Russia, he first needs to admit that he’d lied to the FBI about his own contact with the Russians.

Signs are either Jared Kushner or Trump himself.

Multiple reports on Friday — from Bloomberg, BuzzFeed News, the Associated Press and NBC News — point to Jared Kushner as the person who directed Flynn to contact the Russians. On Saturday, The New York Times reported that emails show Flynn was part of a wider effort by the Trump transition team to reassure the Russians after the Obama administration imposed sanctions in retaliation for meddling in the U.S. election.

Whoever it is, though, Flynn’s plea shows the investigation is making its way up the Trump hierarchy, and could be honing on an obstruction of justice charge against the president himself.

The documents leave a second official unnamed who also knew of Flynn’s Russia connects. The Associated Press reported Friday evening that that person is KT McFarland, the former deputy national security adviser.

READ: Jared Kushner reportedly ordered Flynn to quietly contact the Russians

Flynn’s son, Michael Flynn, Jr, at least as far as we know.

Mueller was reportedly investigating an alleged scheme in which Flynn and his son were to help get a Turkish cleric out of the U.S. and back to Turkey in exchange for $15 million, according to the Wall Street Journal. Flynn, Jr. was reportedly also under the scrutiny of Mueller’s probe.

Flynn had done over half a million dollars’ worth of work as a foreign agent of Turkey, including writing an op-ed in strong support of the country — and against the cleric in question — that was published on Election Day.

It’s not clear yet to what extent Flynn, Jr. was a focus of the investigation, but it’s possible that Mueller could have used the younger Flynn as leverage to earn Flynn Sr.’s cooperation.

Trump has slowly distanced himself from Flynn over the last few months.

After Friday’s hearing, White House lawyer Ty Cobb referred to Flynn as “a former Obama administration official,” and later tried to claim that the Obama admin had approved of Flynn’s contact with Russia, according to CNN.

Former Director of National Intelligence James Clapper reportedly said that was “absurd.”

Here are the charging documents: