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Michael Flynn emerges as a key cooperating witness in the Mueller investigation.

Special Counsel Robert Mueller filed a memo Tuesday recommending leniency towards former National Security Advisor Michael Flynn, arguing that he should receive no jail time for his admitted lying to the FBI. While heavily redacted, the memo makes clear that Flynn has been extremely useful to the Mueller investigation. Flynn has sat for 19 interviews with the special counsels office or with other government attorneys. His example is credited with encouraging other witnesses to come forward. As Jonathan Chait argues, there is ample reason to believe that Flynn is “singing like a canary.”

Making the case for Flynn receiving a light sentence, Mueller writes, “Given the defendant’s substantial assistance and other considerations set forth below, a sentence at the low end of the guideline range—including a sentence that does not impose a term of incarceration—is appropriate and warranted.”

The memo also makes reference to a criminal investigation that is separate from the main inquiry into Russian interference in the 2016 election. Because so much of the text is blocked out, it’s unclear what this criminal investigation concerns.

As Yahoo News reports, “Most of the blacked out portions of the addendum dealt with a criminal investigation that the document distinguished from the special counsel’s principal probe into coordination between Russia and the Trump campaign. While Mueller’s documents provided almost no detail about this mysterious investigation, they confirmed it was separate from the Russia probe and that it is continuing.”

This led to immediate speculation about who could be the target of that criminal investigation:

Other commentators preferred to focus on the visual impact of the redaction: