Of all the avenues in the sprawling Russia investigation, Robert Mueller appears to have advanced the furthest toward assembling an argument that Donald Trump obstructed justice. And in the coming weeks, the special counsel’s team will interview a former top White House aide who could verify key pieces of the puzzle. Then Mueller would confront the thorniest question yet: does he prosecute the cover-up or hold out to try to nail possible underlying crimes?

The core of the obstruction claim has been clear for months. On February 14, 2017, Trump convened eight of his top advisers in the Oval Office for a counterterrorism briefing. That subject, all by itself, made for plenty of tension: courts were rejecting Trump’s first travel ban, and he’d recently fired acting Attorney General Sally Yates for refusing to defend it. But it’s what happened at the end of the meeting that Mueller cares about.

According to the highly detailed, now-famous testimony by former F.B.I. director James Comey, Trump shooed the other seven witnesses out of the room, then leaned on Comey to drop the investigation of Michael Flynn, who the day before had resigned as national security adviser, leaving behind a tangle of dubious Russian relationships. Trump has disputed that account, but what could be important supporting evidence has emerged regularly ever since. In May, Trump fired Comey, then blabbed to NBC’s Lester Holt that the Russia investigation was on his mind when he made the move. In July, Donald Trump Jr. issued a misleading statement to The New York Times about his meeting with a Russian lawyer at Trump Tower—a statement that his father, the president, helped craft. At the beginning of September came reports of a letter Trump drafted to explain Comey’s firing, with rationale that included Comey’s unwillingness to publicly exonerate Trump. “I think there’s a lot of evidence here for obstruction,” said Nick Akerman, a former Watergate prosecutor. “Most recently, you’ve got the reports of Trump repeatedly lambasting Jeff Sessions for recusing himself, which led to the hiring of Mueller. That’s supporting of obstruction too. It shows Trump doesn’t want this thing investigated. The only issue is proving corrupt intent.”

That’s where Reince Priebus comes in. He was Republican National Committee chairman during the 2016 campaign and then became Trump’s White House chief of staff. Comey testified that Priebus was one of the aides shooed out of the Oval Office, and that as Trump pressed his point about Flynn, Priebus poked his head back in through the doorway.

Priebus was fired acrimoniously in July. He has kept a doggedly low profile ever since, in contrast to his fellow Trump exiles. Priebus hasn’t gone on a late-night TV tour like Sean Spicer; or attempted to launch a nebulous Web site like his profane nemesis Anthony Scaramucci; or promoted himself as the mastermind behind a bigoted Alabama Senate candidate like Steve Bannon. Instead, Priebus has quietly played some golf, shopped himself as a paid speaker, and looked for a law firm job. “Reince is fine,” said his longtime friend Brad Courtney, the chairman of Wisconsin’s Republican Party. “We’ve traded some texts. I was really glad that he got to take a nice vacation with his family.”

All of which is in keeping with Priebus’s mild-mannered style as a Republican operative—but it’s one of the things that makes him a potentially dangerous witness: he’s sane and credible. Priebus could confirm whether Trump or Comey is telling the truth about the circumstances of the Oval Office meeting, but also shed light on Trump’s motives for firing Comey. And of the seven people, aside from Trump and Comey, who attended the fateful meeting, Priebus is the only one not still in the employ of the administration.