NEW: Mueller's office says former National Security Adviser Mike Flynn "has assisted with several ongoing investigations, including its probe of coordination between the Trump campaign and the Russian government. 
Mueller's office says that it can't reveal the details of all of Flynn's assistance because "the investigations in which he has provided assistance are ongoing."
Among the matters on which Flynn has provided assistance to the government is a criminal investigation - apparently separate from the collusion probe - that remains totally secret here.
Flynn provided prosecutors with "firsthand information about the content and context of interactions between the transition team and Russian government officials," conversations in the weeks before Trump took office.
Flynn also provided information to Mueller about a second matter that's still secret, this one seemingly under the umbrella of the investigation into links between the Trump campaign and Russia.
Mueller's office says Flynn "began providing information to the government not long after the government first sought his cooperation." And his guilty plea "likely affected the decisions of related firsthand witnesses to be forthcoming with the SCO."
In large part because of his cooperation, Mueller's office says a sentence at the low end of the guideline range (0-6 months) - "including a sentence that does not impose a term of incarceration - is appropriate and warranted."
Mueller's office says Flynn's "record of military and public service distinguish him from every other person who has been charged as part the SCO’s investigation" - a fact that speaks both to his character and suggests he should have known better.
Mike Flynn was the first member of President Trump's inner circle to cooperate with the special counsel investigation. For a year, we've known little about the subjects of that cooperation. Tonight, prosecutors said it was extensive, the cases are ongoing, and it's still secret.
Full story from @usatoday: Mueller probe: Michael Flynn has offered 'substantial' assistance to investigation
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Here's the government's sentencing memo -> documentcloud.org/documents/5425…
And here's the formal motion to reduce Flynn's sentence in exchange for his substantial assistance ->
And here's the formal motion to reduce Flynn's sentence in exchange for his substantial assistance ->
