Paul Manafort’s stunning decision to plead guilty and cooperate with Robert Mueller’s investigation immediately set off speculation about the biggest possible target: what will Manafort tell the special counsel that could incriminate President Donald Trump? But one thing that has become clear during the course of Mueller’s 16 months of work is that he is assembling his case against powerful political figures in the same way he once pursued murderers and mobsters: by moving methodically up the food chain. So while Trump himself probably has plenty to worry about from a newly chatty Manafort, other members of his inner circle could be at more imminent risk, including the first son, Donald Trump Jr.

“Because of Manafort’s cooperation, we will now have additional insight into the Trump Tower meeting, which we all know is hugely consequential,” says Glenn Kirschner, a former federal prosecutor. That infamous June 9, 2016, gathering was the product of an e-mail promising “some official documents and information that would incriminate Hillary” sent by a British music promoter to Trump Jr., who responded, “If it’s what you say I love it.” Days after the meeting, Wikileaks began publishing e-mails that Russian military intelligence had hacked from Democratic servers. The attendees of the Trump Tower meeting included a Kremlin-connected Russian lawyer; Jared Kushner; Trump Jr.; and Manafort. Publicly, they have all since offered varying and benign accounts of what was discussed. Now Manafort will be compelled to describe the meeting in detail and under oath. “Manafort is going to be able to put meat on the bones of that meeting because he’s the guy who had been doing business with Russians and Ukrainians who were being controlled by Russians for a very long time,” Kirschner says. “I don’t know that anybody actually believes when Don Jr. says it was a nothing-burger. That’s coming from a source who cannot be relied upon to tell us what happened because he’s not going to incriminate himself.”

While that single meeting has drawn outsized attention, it came during a period in which Manafort, as campaign manager and then chairman, was in frequent contact with Trump Jr., a top adviser. The intense four-month stretch included Trump clinching the Republican nomination and efforts to make a Republican-convention platform plank more favorable to Russian interests. In August, 2016, Manafort was messily dismissed from the campaign after reports that he’d received dubious payments for work in Ukraine. “The only thing we can know for sure is that this deal means the government believes that Manafort has evidence he can provide that will or could lead to substantial assistance in the prosecution of others,” says Daniel R. Alonso, a former federal prosecutor. “If those others are people in the president’s orbit, I have no idea.”

Earlier this week, Trump Jr. made an appearance on ABC’s Good Morning America, ostensibly to push back against Bob Woodward’s scathing portrayal of the Trump White House and against the anonymous New York Times op-ed writer who declared him or herself to be part of an internal resistance saving the country from Trump. Yet Don Jr. had to fend off a question about whether he fears he will end up behind bars. “I’m not because I know what I did, and I’m not worried about any of that,” he replied evenly. Yet his pile of legal headaches is substantial. Chris Coons, a Democratic senator from Delaware, is pressing for Trump Jr. to return and clarify testimony he gave the Senate Judiciary Committee in September, 2017. “He was asked whether foreign nationals other than the Russians offered to provide assistance to the Trump campaign, and he said no,” Coons says. “Then you had reporting by the New York Times about this specific meeting with an emissary for the crown princes of Saudi Arabia and the U.A.E., as well as a social-media specialist. I think he ought to come back in and answer questions.” A lawyer for Trump Jr. has said Trump Jr. was not interested in the ideas being pitched “and that was the end of it.”