It makes an odd kind of Trumpian sense that what might turn out to be the crucial meeting in Robert Mueller’s investigation of the Trump campaign’s Russia ties is recorded in a commemorative photograph.

That March 31 gathering brought together most of the members of the Trump presidential campaign’s national security/foreign policy committee. At one end of the table sat Senator Jeff Sessions, the head of the committee. At the other sat the candidate himself, Donald J. Trump. In between: George Papadopoulos, who this morning went from obscure conservative operative to crucial collusion witness, when Mueller revealed that Papadopoulos has pleaded guilty to lying to F.B.I. agents and is cooperating with the special counsel’s investigation.

Papadopoulos has been a bit player in collusion journalism for months. In August, The Washington Post broke the news that Papadopoulos had sent campaign officials an e-mail with the subject line, “Meeting with Russian Leadership—Including Putin.” The money shot of that piece was that campaign chairman Paul Manafort rejected a proposal for Trump to take the meeting. Business Insider had also pointed to Papadopoulos’s possible importance. Yet the dots have been most persistently connected by Seth Abramson, a former public defender and current University of New Hampshire English professor, who has been tweeting long threads about Papadopoulos for weeks. In various posts, he’s highlighted the March 31 meeting as the moment when Papadopoulos seems to have announced to Trump and Sessions that his Russian connections were eager to aid the campaign. But even Abramson was surprised by the revelations in the plea document that Papadopoulos claims to have been offered “dirt” on Hillary Clinton from a man he believed to be a Russian professor and a woman who claimed, falsely, to be Vladimir Putin’s niece.

“From his conversations with this ‘Russian professor’ and this unnamed female Russian national, all of which preceded March 31, we know Papadopoulos was developed by the Kremlin at a time they knew he worked for the Trump campaign,” Abramson says. “We know that on March 31, Papadopoulos revealed himself to be—in the legal sense, not in the movie sense—a Kremlin agent, because they had authorized him to be their liaison in setting up meetings between Trump campaign officials and Kremlin officials.” One corroboration of Papadopoulos’s actions in the national security meeting that day comes from an August 17, 2017, story in, of all places, the conservative Daily Caller—which reported that Sessions told Papadopoulos to drop the subject and not bring it up again. Instead Papadopoulos, in the ensuing weeks, sent out e-mails to Trump’s national security team pursuing the idea, leading to discussions about whether or how such a meeting would work. Another reason the March 31 meeting is important: committee member J.D. Gordon told CNN’s Jim Acosta that he successfully pushed to change Republican platform language, making it more Russia-friendly, because of what Trump had said that day.