Over the course of special counsel Robert Mueller’s investigation into the Trump campaign’s ties to Russia, Mike Pence has perfected a disappearing act that is almost incomprehensible in its precision. The vice president has said he did not know about Michael Flynn’s foreign lobbying or his conversations with Russian Ambassador Sergey Kislyak. He had no comment when Flynn pleaded guilty to lying—to him, among others—about these conversations. And his office has repeatedly noted that, at crucial points in the Russia timeline, Pence was back in Indiana serving soup to the homeless or attending his son’s wedding. Despite his efforts to shield himself from the scandal, however, Mike Pence’s knowledge—or lack thereof—has emerged as a critical piece in Mueller’s Russia puzzle.

As NBC News reported Monday, Mueller appears to be building an obstruction case centering on the two-week period in the chaotic early days of the Trump administration. On January 26, one week after the inauguration, acting Attorney General Sally Yates warned White House general counsel Donald McGahn, among others, that Flynn had lied to the F.B.I. about his call with Kislyak. As we now know, Yates was concerned that Flynn had made himself vulnerable to blackmail by promising the Russians that the incoming Trump administration would pursue a more accommodating sanctions regime. Flynn had told officials—including Pence—that he did not discuss U.S. sanctions on that call, a claim which turned out to be false. It would later emerge that he had also lied to federal law enforcement officers about the same. Yates was fired on January 30, ostensibly for refusing to enforce Trump’s travel ban. Flynn, despite the allegations against him, wasn’t fired until February 13.

Mueller’s goal in scrutinizing this juncture, according to a handful of people who the special counsel has interviewed, is to determine whether the White House sought to bury the information that Yates had presented. Although the White House received a half dozen warnings about Flynn, the president appears not to have moved to fire Flynn until The Washington Post published a report making Yates’s warning public. According to her Senate Intelligence Committee testimony in May, Yates followed up with McGahn on January 27, but the White House general counsel didn’t believe that Flynn lying to Pence was problematic. “Essentially: ‘what’s it to the Justice Department if one White House official is lying to another?’” she said he asked. During an interview with NBC’s Lester Holt in May, Trump said he did not ask for Flynn’s resignation after Yates’s tipoff because McGahn “came back to me and [it] did not sound like an emergency.”

But the delay has raised eyebrows among legal experts. Two former prosecutors who spoke to NBC on the condition of anonymity said most White House general counsels would have gone directly to Flynn to ask whether he lied to F.B.I. investigators. And Justice Department officials told the outlet that they expected Trump to fire Flynn immediately after Yates informed McGahn that he had lied to senior administration officials. At the time, Trump said he fired Flynn because the national security adviser lied to Pence. But the weekend after Flynn pleaded guilty to lying to the F.B.I. about his Russian contacts, Trump gave a different reason on Twitter: that he fired Flynn because Flynn lied to the F.B.I.—a piece of information that the president had not previously disclosed. (Trump’s attorney John Dowd later claimed that he penned the potentially damning tweet.) Two sources familiar with Mueller’s probe told NBC that Mueller is seeking to determine why Flynn remained in his post as long as he did, whether Trump was aware that he had lied to the F.B.I., and if he possibly instructed Flynn to mislead other officials, including Pence.

When Trump fired James Comey, who was investigating Flynn, he had the full legal authority to do so. The question, as members of the Washington bar have told me, is whether he did so with corrupt intent. Mueller doesn’t necessarily have to uncover an underlying crime to prove that Trump or others obstructed justice, but only that the White House attempted to conceal something. As Renato Mariotti, a former Illinois federal prosecutor, told my colleague Chris Smith, “What matters for obstruction is what Trump knew—or thought he knew—about Flynn’s liability, not what Flynn’s liability actually was. Even if Flynn would have gotten off scot-free but Trump was convinced that Flynn was in deep trouble, and acted because of that belief, then Trump has a problem.”