For the Russia investigation, the filing said Flynn provided "content and context" of interactions that the Trump transition team had with Russian officials regarding sanctions that the Obama administration sought to impose at the end of 2016, as well as a vote on a United Nations Security Council resolution about Israel.
Flynn, whose sentencing is scheduled for Dec. 18, pleaded guilty in December 2017 to lying to the Federal Bureau of Investigation about his conversations with then-Russian Ambassador Sergey Kislyak during the presidential transition period from election day 2016 to Trump's inauguration in January 2017.
Mueller indicated that Flynn had already participated in 19 interviews in the criminal investigation.
The recommended sentence, or lack thereof, provided the federal judge overseeing Flynn's case with a description of how valuable he has been to Mueller's probe and comes just two weeks before the former adviser's December 18 sentencing date.
Last week Michael Cohen, Trump's former legal fixer, said he had lied to Congress about Trump's plans to build a tower in Moscow, in an attempt to protect the president. One knowledgeable source said the bulk of Flynn's cooperation with the special counsel occurred immediately following his December court appearance.
Trump Signs New North American Trade Pact at G20
Trump could threaten to unilaterally pull out of NAFTA if the new deal fails to get through Congress. Mexican and Canadian officials had strongly objected to the tariffs when Trump imposed them June 1.
Mueller's office did praise Flynn's character, saying his "record of military and public service distinguish him from every other person who has been charged as part the investigation".
Mueller's team has entered plea agreements with several Trump associates, including former Trump campaign chairman Paul Manafort, who the special counsel now alleges breached their plea deal by lying to investigators.
He then campaigned vigorously for Trump in 2016, encouraging audience members during his speaking turn at the Republican national convention in Cleveland, who were chanting "Lock her up!" when he mentioned Hillary Clinton's name. He has admitted he lied to Congress to help Trump and broke campaign finance laws by helping to arrange hush money for women who might have embarrassed Trump with accusations of infidelity as he was running for president.
Whatever it is that Flynn said, it's clear that Mueller liked it.
His sentencing was delayed for several months but even close confidantes have told ABC News they don't know why.
The special counsel concludes by noting that Flynn's early guilty plea, "likely affected the decisions of related firsthand witnesses to be forthcoming with the [special counsel's office] and cooperate". "It is a shame because his actions during the transition were lawful", Trump tweeted after Flynn pleaded guilty a year ago.