Cohen has twice pleaded guilty in federal court in Manhattan to a litany of crimes, and he has volunteered information to the special counsel and other agencies investigating Trump and his inner circle.
He has not explained his legal approach since he first pleaded guilty in August.
The news is not entirely unexpected. Assuming each redacted portion was in reference to at least one investigation, that means that Flynn helped with at least two unknown investigations besides Mueller's Russian Federation probe.
Prosecutors said the two men discussed USA sanctions against Russian Federation and that Flynn also asked Kislyak to help delay a United Nations vote seen as damaging to Israel.
Flynn served as Trump's national security adviser for less than a month in early 2017, but was sacked more than two weeks after then-acting Attorney General Sally Yates told White House officials that Flynn had lied to Vice President Mike Pence about his communications on Russian sanctions with then-Russian ambassador to the US Sergey Kislyak.
In addition, the Mueller probe has brought down indictments on over two dozen Russians related to cyber intrusions into USA affairs, though they will likely remain out of the grasp of prosecution as the US has no extradition treaty with Moscow.
The detailing of Flynn's cooperation also comes as Trump has increasingly vented his anger at the probe - and at one of his former confidantes who cooperated with it.
Department of Justice veterans told INSIDER that they had never heard of a cooperator maintaining contact with the potential target of an investigation after striking a deal with prosecutors and that such communications could tank a cooperation agreement.
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The memo extends to seven pages, but numerous details were redacted from public view.
Flynn's case has been a contrast to those of other Trump associates, who have aggressively criticized and tried to undermine the Russian Federation probe.
Trump has tweeted in the past in support of Flynn, saying that Mueller had "totally destroyed" his life unfairly.
Robert Muller says Michael Flynn should get no prison time because of the "substantial assistance" he has provided in the Russian Federation probe. The sentencing is scheduled on December 12. Not only did he draw notice to how soon into the investigation he agreed to cooperate, but he also pointed out how valuable his cooperation has been.
The sentencing memo was "respectfully submitted" by special counsel Mueller himself.
The filing included an addendum with some redactions that goes into more specifics about his a cooperation.
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