As NBC reports: “Trump’s legal team and senior White House aides are refusing to say when and how the president first learned that Flynn had lied to the FBI.”

After the news broke that Flynn had made a plea deal admitting to these lies, Trump tweeted that he’d fired Flynn because he lied to the FBI, suggesting he knew of this lying at the time, though his attorney has since insisted that this isn’t what he meant. This has raised questions as to why Trump would press his former FBI director to drop the investigation into Flynn (as James B. Comey has testified, and Trump denies) in the apparent knowledge that he’d lied to the FBI, which could constitute obstruction of justice.

But beyond that important question, there is also the question of what Trump knew about the lying itself and when. And Mueller is looking at this, per NBC:

Mueller is trying to determine why Flynn remained in his post for 18 days after Trump learned of Yates’ warning, according to two people familiar with the probe. He appears to be interested in whether Trump directed him to lie to senior officials, including Pence, or the FBI, and if so why, the sources said.

If Trump knew his national security adviser lied to the FBI in the early days of his administration it would raise serious questions about why Flynn was not fired until Feb. 13, and whether Trump was attempting to obstruct justice when FBI Director James Comey says the president pressured him to drop his investigation into Flynn.

Bob Bauer, a former White House counsel, emailed me this about the NBC report:

“This is a potentially serious development in this investigation. Should there be evidence that the president directed or encouraged Flynn to lie, he faces an obstruction charge, and the constitutional defenses his supporters have been claiming are irrelevant. Of course, this legal exposure extends to any other officials who were involved in a decision to have Flynn make these false statements.”

That line about “constitutional defenses” is a reference to an argument that prominent Trump supporters have made: That Trump cannot by definition obstruct justice if he is simply exercising his constitutional authority, as he was when he fired Comey. That argument is itself questionable, since, as some experts have noted, he might have done this with corrupt intent. But beyond this, directing Flynn to lie to the FBI would not constitute such a legitimate exercise of authority, and could constitute obstruction of justice.

“This would mean we’re now looking at potential criminality that cannot be justified as an exercise of the president’s authority,” Paul Rosenzweig, a senior counsel on Ken Starr’s investigation into Bill Clinton who is now a lecturer in law at George Washington University, told me today. “That could be obstruction of justice, or aiding and abetting a false statement to the FBI, or conspiracy to do the same.”

Another possibility, Rosenzweig noted, is that Trump or other top officials may have merely may have been made aware of Flynn’s lying soon after the fact. If so, the question would be why they did not try to correct the record with the FBI, which might not be criminal but could potentially be “impeachable,” Rosenzweig says. A third possibility, he says, is that Trump or top officials tacitly approved of this lying beforehand, which could constitute “a conspiracy to obstruct justice.” The bottom line, as Randall Eliason, a professor of white collar criminal law at GWU, told me, is that if the NBC story is true, “Mueller may be looking at possible obstruction of justice by Trump.”

To be clear, it is possible that Flynn freelanced in lying to the FBI, and that no one else either directed him to do it or was made aware of it at the time. So this could end up meaning little.

Time will tell.