George Papadopoulos, who served as a foreign policy adviser to the Trump campaign, secretly pleaded guilty three weeks ago to charges that he lied to federal agents about meetings he had with Russians linked to the Kremlin during the 2016 presidential campaign.

This revelation by the Justice Department on Monday came a few hours after the DOJ unsealed the indictment of former Trump campaign chair Paul Manafort and his longtime business associate on charges of money laundering, tax evasion, and other charges.

But while the indictment of Manafort made the biggest splash, it’s the Papadopoulos guilty plea that could prove far more damning to President Trump.

According to the special counsel’s statement of the offense, Papadopoulos joined the Trump campaign in March 2016 as a foreign policy adviser. That month, he met with a Kremlin-linked professor in London who also introduced him to a woman possibly close to Russian President Vladimir Putin. When the FBI interviewed Papadopoulos as part of the investigation into Russian interference in the 2016 election, the document says, he lied to FBI agents.

Papadopoulos was arrested on July 27 and pleaded guilty, and a special counsel document dated October 5 says he agreed to plead guilty. However, special counsel Robert Mueller asked the court to keep all of that secret until now because, in Mueller’s words, the “[d]efendant has indicated that he is willing to cooperate with the government in its ongoing investigation into Russian efforts to interfere in the 2016 presidential election.”

“Public disclosure of the defendant’s initial appearance, however, would significantly undermine his ability to serve as a proactive cooperator,” he continued.

In other words, Mueller wants him to offer and gather up more information about possible Trump-Russia ties. It’s worth noting just how impressive it is that Mueller and his team kept all of these developments secret until they chose to release the information today.

Lawyers for Papadopoulos are not discussing the matter right now. “We will have the opportunity to comment on George’s involvement when called upon by the Court at a later date,” a press release from his counsel said.

But while Manafort may have the spotlight, Papadopoulos may actually end up as Mueller’s biggest get so far. That’s no small feat for an investigation that the president claims is just Democrats’ excuse for losing the election.

Papadopoulos interacted with Russians. A lot.

Here’s what happened, according to court documents.

In March 2016, Papadopoulos, a young, self-described oil and energy expert, was hired as a foreign policy adviser for the Trump campaign. Shortly after, he met with a London-based professor with ties to Russia who took interest in him because of his role in Trump’s campaign. That professor then introduced Papadopoulos to a Russian woman described as a relative of Putin. Further, the professor introduced Papadopoulos to an individual with connections to the Russian Ministry of Foreign Affairs.

These relationships piqued Papadopoulos’s interest, especially since the connections could help him be a more influential figure in the campaign. He continued to liaise with his newest contacts with the express goal of setting up a meeting between members of the Trump campaign and Russian officials.

And then in late April 2016, the professor told Papadopoulos that the Russians had “dirt” on Hillary Clinton in the form of “thousands of emails.” The special counsel’s documents don’t mention if Papadopoulos ever discussed the “dirt” with leaders in the Trump campaign. It’s worth pointing out, though, that the professor relayed that information before the public found out that the Russians had hacked the Democratic National Convention and released its emails over the summer.

When FBI agents questioned Papadopoulos about all of this in January, he lied to them and left out critical information. He told agents that the professor was “a nothing” and "just a guy talk[ing] up connections or something," when in fact Papadopoulos knew that the professor had Kremlin-linked connections. He also claimed that his emails with the Russian woman consisted of nothing more than messages such as “Hi, how are you?” But Papadopoulos clearly tried to leverage her and his other contacts to get a Trump-Russia meeting.

Lying to federal officials is a crime, and Papadopoulos was subsequently charged. But now it appears he is helping the Mueller probe — and that could spell trouble for Trump.

Papadopoulos might be working against the president

Mueller’s team describes Papadopoulos as a “proactive cooperator.” That’s a big deal.

Here’s why: Mueller purposely sealed the indictment and kept the arrest secret so that others wouldn’t know Papadopoulos was working with his team — because the probe might be using Papadopoulos to obtain even more information on possible Trump-Russia collusion.

The Toronto Star’s Daniel Dale reports that when prosecutors consider someone to be a “proactive cooperator,” it could signal that that person was wearing a wire. And if that’s true, that means Papadopoulos might’ve talked to Trump campaign officials with a wire on. That’s still speculative, of course, but it could pose a serious problem for Trump if officials with secrets to keep unknowingly divulged information to a wired-up Papadopoulos.

That would be an interesting development for someone who was once in Trump’s inner foreign policy circle, especially since Papadopoulos features in a photo Trump tweeted in March 2016 of his foreign policy team (he’s on the left side one seat away from Sessions).

But what seems clear is that, as NBC’s Ken Dilanian notes, what happened to Papadopoulos was a classic way for Kremlin-connected individuals to infiltrate the Trump campaign. The game plan is to latch onto a low-level staffer, meet with him or her, and use him or her to set up higher-level contacts in the campaign.

So it’s possible that Papadopoulos also has a lot to tell Mueller’s team about how Kremlin-tied people tried to finagle their way into the heart of Trumpworld.

It’s an amazing story that appears to come out of a bad spy thriller, but it’s real. And if Mueller could keep all of this secret for so long, who knows what other publicly unknown information he has in his back pocket.