Mueller offers other enticing clues about the nature of those ongoing investigations. It appears that Flynn has contributed to three distinct investigations: (1) an investigation of "interactions between the [Trump] transition team and Russian campaign officials," including the dealings of Flynn and potentially others with Kislyak over sanctions; (2) Mueller's core "investigation concerning any links or coordination between the Russian government and individuals associated with the campaign of President Donald J. Trump;" and (3) another mystery case.
Mueller's description of the mystery case, at section I.A of part two of the submission, begins with "The defendant has provided substantial assistance in a criminal investigation" -- followed by 22 lines of black redaction ink. There simply is no way to know who or what this investigation is about. But we do know that it is separate from the investigation of Russian election interference; that it is ongoing; and that Flynn provided information to Mueller about it. If Mueller deemed the mystery case worth including in Flynn's sentencing memo, it likely is an important one.
Mueller then provides some hints about Flynn's cooperation on the Russian election interference investigation. First, we now know that Flynn did have at least some information about Russian election interference and "links of coordination between the Russian government and individuals associated with the Trump campaign." That in itself is noteworthy: Flynn, who held influential and high-ranking positions in the Trump campaign and the early days of the administration, has provided Mueller with information about collusion between Russia and the Trump campaign.
Just as the reader begins to lean in to read the details of Flynn's cooperation on election interference, however, Mueller offers up 31 lines of redaction interrupted only by the phrase "[t]he defendant also provided useful information concerning." So we do not know precisely what Flynn has told Mueller about collusion between Russia and the Trump campaign -- but we know he has told Mueller something, and likely quite a bit.
Mueller has shown a knack for conveying much important information in few words. Those 31 lines of redactions from Mueller could obscure extensive information about Russian election interference, and could spell the death of Trump's "no collusion" refrain.
Mueller offers one final clue in his closing paragraph when he notes that Flynn's cooperation "was particularly valuable" because he provided "long-term and firsthand insight regarding events and issues under investigation" by Mueller's team. The word "firsthand" is important because it means that Flynn was a direct eyewitness to the events he described to Mueller -- whereas less valuable cooperators sometimes offer only secondhand, hearsay or background information. Flynn was actually in the room for the events he described to Mueller, and not merely a second-tier witness peddling rumors and say-so.
So we do not yet know everything about Flynn's cooperation and indeed the heavy redactions hide nearly all the details that Mueller provided to the court. But, if you read around all that black ink, Mueller did say enough to let us know this: Flynn gave up the goods, and Mueller's work is far from over.