Michael Flynn has signed a plea deal with Robert Mueller. Trump should be very worried.

Former National Security Adviser Michael Flynn agreed to a plea deal with prosecutors in special counsel Robert Mueller’s Russia probe. It’s the biggest development yet in the investigation into potential collusion between the Trump campaign and Russia — and the legal move that poses the most direct threat to the Trump presidency itself.

Flynn pleaded guilty to a single count of lying to the FBI on or around January 24 about conversations with then-Russian Ambassador Sergey Kislyak in December 2016, the Office of the Special Counsel announced Friday. It’s important to note that he did not admit to colluding with Russia during the 2016 presidential election.

Ty Cobb, a White House lawyer, immediately tried to minimize the importance of Flynn’s plea deal. “Nothing about the guilty plea or the change implicates anyone other than Mr. Flynn,” he said in a statement on Friday.

But the plea deal will strengthen Mueller’s sprawling probe into Trumpworld’s possible criminal acts and ties with Russia. In October, Mueller charged Trump campaign manager Paul Manafort and former deputy campaign manager Rick Gates with money laundering and failing to properly disclose their lobbying work on behalf of foreign clients. Mueller also unsealed a guilty plea in October from former Trump foreign policy adviser George Papadopoulos, who admitted to lying to federal investigators about meetings in which he discussed potentially colluding with Russian agents to acquire Hillary Clinton’s private emails.

The Flynn plea deal is the most significant moment in Mueller’s probe to date. Flynn is the first person who had actually served in the Trump White House to admit he broke the law. And he wasn’t just any old official: His role as national security adviser is one of the highest-level and most powerful posts in Washington, and the retired three-star general temporarily had enormous sway over Trump’s early policy and personnel choices.

Due to his unique ties to both the Trump campaign and the Trump White House, Flynn is particularly well-suited to answer the two central questions in the Mueller probe: Did the Trump campaign knowingly collude with Russia, and did Trump obstruct justice by trying to limit or derail the FBI’s investigation? A plea deal would get Mueller closer to answering those questions.

The announcement comes after weeks of speculation that Flynn was looking for a way to protect himself from a more serious criminal indictment. The New York Times reported on November 23 that Flynn’s lawyers told Trump’s legal team they could no longer share information, a move that signaled Flynn wanted to work with Mueller. And on November 27, ABC News reported that Flynn’s lawyers met with Mueller’s team, which was the strongest sign that a plea deal was imminent.

There were signs early on that Mueller had a solid case against Flynn. Flynn offered to testify earlier this year in exchange for full immunity from prosecution, but Mueller refused the deal. Now Mueller gets Flynn to talk and also an admission of guilt.

That’s of course bad news for Flynn. But it could potentially be even worse news for Trump.

“When you flip somebody, you’re using them to go up the chain,” Asha Rangappa, a legal expert at Yale’s Jackson Institute for Global Affairs, said in an interview before Flynn agreed to a plea deal. “This suggests that Mueller’s investigation is going to go into the even-tighter inner circle of the campaign and possibly the administration.”

A retired lieutenant general who had served in the Army for more than 30 years, Flynn was named head of the Defense Intelligence Agency, the Pentagon’s intelligence arm, in 2012. There, he clashed with other Obama administration officials, who viewed him as sloppy with facts and incompetent at management. He was soon pushed out, and stepped down from his post in 2014. By all accounts, he was furious.

Out of government, Flynn began commenting on foreign policy and military issues in the media, distinguishing himself with extreme rhetoric about Islam. In February of last year, to take one representative example, he tweeted, “Fear of Muslims is RATIONAL.” This kind of language, combined with his poor track record at DIA, made Flynn a pariah in the mainstream foreign policy community — but a perfect fit for a Trump campaign that was championing policies like a ban on Muslim immigration.

In the fall of 2015, Flynn began occasionally briefing then-presidential candidate Donald Trump on foreign affairs, and his involvement in the campaign gradually deepened. By late May 2016, he was mentioned in the press as a potential vice presidential pick for Trump, and in July of that year he gave a now-ironic speech at the Republican convention in which he said (in response to crowd chants) that Hillary Clinton should be locked up.

At the time that Flynn was advising the campaign, he was operating a lobbying and consulting firm called the Flynn Intel Group — which, crucially, also employed his son. He also was a frequent guest on RT, the Russian government’s English-language propaganda outlet, where he would often espouse the view that the US and Russia had a shared interest in teaming up against Islamic extremism.

It’s these activities — lobbying and work for the Russian government — that appear to have first led Flynn into dangerous legal territory.

In December 2015, Flynn traveled to Moscow to attend a gala celebrating RT’s 10th anniversary. He sat next to Putin himself, and delivered a speech to the attendees about his vision of foreign policy. RT paid a $45,000 speaker’s fee for Flynn’s services; the former general was also paid a total $22,500 by Russian companies for speeches during the same trip.

That in and of itself isn’t necessarily illegal. However, Flynn reportedly lied about the source of the payments in his security clearance renewal form, saying they came from “US companies.” Lying on this form is considered the equivalent of lying to federal investigators, which is a felony — and may be one of the reasons Flynn took the plea deal.

Furthermore, in August 2016, Flynn’s consulting firm was hired by something called Inovo BV — a Dutch company that turned out to be a shell corporation for a wealthy member of the Turkish government. Flynn appears to have continued working for Turkey until November at the earliest, and was paid at least $530,000 by Ankara, Turkey’s capital. (He oddly published an op-ed on Turkey policy in the Hill on election day, without any disclosure.)

Under the Foreign Agent Registration Act (FARA), Flynn needed to publicly disclose any lobbying work he did with the Turkish government when he started doing it. At the time, Flynn’s FARA paperwork said he had worked for a Dutch company — not the Turkish government. In March, Flynn filed paperwork correcting this error — admitting that Inovo was really paying him to work on behalf of Turkish interests.

If that’s all he did, then he probably would’ve been fine: The US government generally doesn’t arrest people for filing incorrect FARA paperwork after they correct it. But if there is more undisclosed lobbying for foreign governments — more Turkey payments, or undisclosed activity for Russia — than he disclosed in March, then he potentially would’ve faced serious legal trouble.

You’d think that Flynn’s questionably legal behavior would have ended in November, when Trump announced that he would serve as national security adviser in the new administration. That’s especially true since outgoing Obama officials warned the Trump transition team about appointing Flynn. But if anything, it got worse.

Throughout the transition, Flynn had several contacts with Kislyak, the Russian ambassador. In one early December meeting at Trump Tower, he and Jared Kushner talked to Kislyak about setting up a secret channel through which they could communicate, according to the Washington Post.

Then on December 29, 2016, the day President Barack Obama announced sanctions on Russia in response to the country’s hacking efforts, Flynn and Kislyak reportedly exchanged five phone calls, and they discussed the topic of sanctions. But Flynn reportedly told Vice President-elect Mike Pence and others on the Trump team that sanctions hadn’t come up in the calls, spurring them to make false statements to that effect in public. This conversation between Flynn and Kislyak is part of the just-released document the special counsel sent to the court.

In the first week of the Trump presidency, Flynn was questioned by the FBI, in which he reportedly denied contact with Kislyak during the transition. In the same week, then-acting Attorney General Sally Yates warned the White House that intelligence showed Flynn had been misrepresenting his conversations with Kislyak, and that he was vulnerable to Russian blackmail. The White House did nothing about this until it leaked to the press a few weeks later, when they were spurred to fire Flynn on February 13.

Then there’s an entirely separate matter of whether Flynn improperly acted on Turkey’s behalf during the transition or while in office. The Wall Street Journal has reported that Mueller is investigating “an alleged plan” in which Flynn and his son would be paid as much as $15 million for forcibly removing Fethullah Gülen, a Muslim cleric living in Pennsylvania, from the United States and delivering him to Turkey. Per the Journal, Flynn discussed this possibility with Turkish government representatives at a December meeting during the transition, while he was the incoming national security adviser.

Altogether, there was a lot of circumstantial evidence that Flynn broke the law. The plea deal where he will admit to lying to federal investigators confirms he did, and he’s now trying to minimize the punishment — and the best way he can do that is tell Mueller everything he knows about Trump and Russia.

The next important question, but the one on which there’s been the least solid evidence, is whether other members of the Trump campaign aided Russian efforts to interfere with the 2016 presidential campaign. If there was collusion, the thinking goes, Flynn very well may have known about it.

That’s probably why Mueller wanted Flynn to strike a deal — one where Flynn likely agreed to a lesser sentence in exchange for giving an honest accounting of what he knows about Trump-Russia ties. Recall that in October the special counsel revealed that George Papadopoulos, a foreign policy aide in the Trump campaign, agreed to cooperate with Mueller’s team. But getting Flynn to likely cooperate is a much bigger prize.

And it’s possible that Flynn has even more Russia ties than is known, since there’s already some reporting that suggests we don’t have the full story when it comes to Flynn and Russia. The Wall Street Journal’s Shane Harris reported in June that Peter Smith, a Trump-supporting GOP operative and private equity executive, embarked on an effort to track down Hillary Clinton’s infamous 30,000 or so deleted emails during the fall of 2016 — and contacted Russian hackers to ask if they had them.

Smith was not part of Trump’s campaign. But according to sources interviewed by Harris, Smith told people working with him that he was coordinating with Michael Flynn, Trump’s main campaign foreign policy adviser.

While trying to recruit for the effort, Smith also distributed a document naming the Trump campaign as one of four groups involved, per the Journal.

Another piece of information pointing toward Flynn, Harris reported, was that US officials were aware of some intelligence that Russian hackers were at least discussing sending leaked emails to Flynn through a third party. He wrote:

Investigators have examined reports from intelligence agencies that describe Russian hackers discussing how to obtain emails from Mrs. Clinton’s server and then transmit them to Mr. Flynn via an intermediary, according to U.S. officials with knowledge of the intelligence.

Smith died this year, reportedly by his own hand, and Flynn hasn’t commented on the Journal report. Still, all of this is enough to raise serious questions about just what Flynn knew about this or any other attempted outreach to Russian hackers or other Russian entities.

But we don’t yet know if this led to any actual collusion implicating Flynn or anyone on the Trump team. It’s at least possible that Smith was just trying to make his effort seem important by name-dropping Flynn, rather than actually working closely with him. Furthermore, Smith’s efforts to find Clinton’s deleted emails appear to have failed, since the emails never surfaced.

So this could be an issue of vital significance, or it could be nothing — and Mueller is about to find out which one it is.

Somewhat separate from the question of collusion is the question of whether Trump committed obstruction of justice after taking office, essentially by unlawfully interfering with former FBI Director James Comey’s inquiry. Flynn is a central character in the entire drama — and his fate could prefigure Trump’s.

“I think Flynn’s value to Mueller is less on the collusion part and has more to do with obstruction of justice,” Rangappa told me before the plea deal announcement. “If Trump had any knowledge of any kind of criminal liability that Flynn may have had — and he was trying to get Comey to drop the investigation — that essentially seals Mueller’s obstruction case.”

Flynn was fired on February 13. The next day, Trump held a counterterrorism meeting with his remaining national security officials — which ended when he ordered everyone except then-Director Comey to clear the room. According to Comey’s written notes, Trump then asked him to lay off the FBI investigation into Flynn’s Russia statements, urging Comey “to see your way clear to letting this go, to letting Flynn go.” Comey refused to make that promise, and Trump eventually fired him.

Whether the president wanted Flynn protected purely out of friendship, or because he feared what Flynn might know, is something Flynn’s testimony could help answer either way.

It’s not enough, former federal prosecutor Alex Whiting wrote in July for the legal site Just Security, to show that Trump believed the Russia investigation was going in the wrong direction. A prosecutor or member of Congress pushing for impeachment would need to show that Trump was actually trying to cover up some kind of wrongdoing on his own part to establish an obstruction case.

And the specifics of Trump’s relationship with Flynn, Whiting explained, matter a great deal on this point:

Did [Trump] know that Flynn’s story was an important piece in the larger picture, one that he did not want revealed? Or did he know that the FBI’s pressure on Flynn could force him to give up other incriminating evidence? Far from simply acting to shield a former subordinate and ally, was Trump actually just trying to protect himself, and those close to him?

If the answer to any of these questions is yes, then Trump’s actions will have a very different feel to them, and his potential defenses much harder, if not impossible, to swallow.

Flynn may very well know the answer to Whiting’s questions. Mueller will likely hear those answers soon.