While there’s been plenty of thoughtful legal analysis of the sentencing memo for former National Security Adviser Michael Flynn, one big nugget has been all but overlooked. And it may answer a long-standing key question surrounding Flynn’s role in the Russia investigation.

On page 3, Mueller is indicating that Flynn’s phone calls with Russian Ambassador Sergey Kislyak during the transition were part of a quid pro quo: pulling back sanctions on Russia in exchange for their help during the election.

Here is the key passage:

Crucially, Flynn’s lies were “material” to the investigation into Trump campaign ties to the Russians (not transition ties). Mueller is very clear that Flynn’s wrongdoing took place during the transition—not during the campaign. He’s specifying “campaign” here because the horse trading between Flynn and Kislyak (the “requests he conveyed” and “Russia’s response to the requests)” was tied to the campaign.

This links Flynn’s phone calls, during which he coordinated with Kislyak to undermine the Obama administration’s sanctions on Russia, to what happened before: Russia’s support for President Donald Trump during the campaign.

This is the strongest evidence yet of a quid pro quo: It suggests Flynn’s efforts to undermine or roll back sanctions on Russia, which continued into his brief tenure as National Security Advisor, were Russia’s reward for helping Trump win.

It seems that Flynn didn’t lie because he was trying to cover up his actions during the transition; he lied because telling the truth would have exposed the Trump campaign’s collusion with the Russian government.

According to the old cover story, Flynn: talked with Kislyak and urged Russia to not retaliate against Obama administration measures in a vague discussion about sanctions and; when his calls were discovered, Flynn lied to the Vice President about it, then; he supposedly perpetuated this lie when confronted about it by FBI agents on his third day in the job, for reasons unknown to anyone.

The story never made much sense… because it’s probably not true.

Mueller’s filing also shows Flynn was even more compromised than previously believed: Russia knew Flynn’s calls were part of Putin’s payoff for colluding during the campaign, giving the Kremlin extraordinary leverage over the national security advisor.

So did Flynn use his position as National Security Advisor to facilitate the quo for Russia’s quid? Mueller is indicating that was indeed the case. And this is the latest in a pattern of filings that show Mueller is no longer investigating whether collusion occurred; he’s identifying how deep that collusion went.

Somebody who should be very worried right now: Vice President Mike Pence, who headed Trump’s transition team.