The filing also makes clear that Flynn's lie - which, to be clear, he has pleaded guilty to - wasn't a mere slip-up in a casual interview.
"While the circumstances of the interview do not present mitigating considerations, assuming the defendant continues to accept responsibility for his actions, his cooperation and military service continue to justify a sentence at the low end of the guideline range", he said.
It now seems there might have been another reason for the long sentencing delay: The Mueller team was desperately trying to figure out how to avoid having to hand over these exculpatory documents to Flynn's lawyers and giving away the entire entrapment scheme.
The disagreement is unlikely to affect Flynn's chances for probation, but it's attracted the attention of President Donald Trump, who said this week that Flynn did not lie despite having fired him almost two years ago for just that reason.
They contrasted Flynn's case with that of Trump campaign aide George Papadopoulos and Alex van der Zwaan, a lawyer who worked with Trump campaign associates, who have pleaded guilty to lying to federal authorities and were both informed of the seriousness of the crime.
Mueller's filing provides numerous new details about Flynn's interactions with Kislyak shortly after the 2016 election.
During the interview, Flynn denied asking Kislyak that Russian Federation refrain from reacting harshly to sanctions imposed by the Obama administration over Russia's campaign of disruption and said he did not remember Kisklyak saying that Moscow had backed off as a result of Flynn's request.
Among Flynn's fans is former Trump national security aide Sebastian Gorka, who worked with Flynn in the White House.
"To come in and say 'what did you talk to Sergey Kislyak about, ' and then you lie about it, that isn't entrapment".
"I replied that we were quite concerned about what we perceived as significant leaks", wrote McCabe. "I assume he would know what he's doing".
According to McCabe's memo, Flynn knew going into the White House interview that his conversations with Kislyak would be a topic of conversation.
He adds: "Number one, it wasn't a campaign contribution". Strzok was sacked this summer. But Comey described that description as a "garble" in a private interview with House lawmakers last week.
Flynn, however, agreed to be interviewed alone. The quickest way to "get this done" was to just meet directly with the agents.
Flynn was among the first people charged in the 19-month-long probe into Russian interference in the 2016 USA presidential election.
"Flynn was alone and "relaxed and jocular", according to the Strzok document.
President Trump was too busy directing workers where to place art in the West Wing on January 24, 2017 to notice the two Federal Bureau of Investigation agents walking by whose impending interview of his national security advisor was about to dog his presidency for years.
"Nobody paid attention to the agents", the account of the episode said. He said the president directed him to make hush money payments to women during the 2016 campaign.
Mueller, in court papers filed Friday ahead of Flynn's December 18 sentencing, rebuts an implication by Flynn's attorneys this week that the agents lulled the onetime Trump aide and U.S. Army general into a false sense of security by playing down the severity of the situation and failing to warn him that lying to them would be a crime.