But his lawyers, in a sentencing memorandum filed late Friday night, offered the fullest explanation yet for Cohen's motivation in switching sides and joining the government team, as prosecutors like to say.
Mueller's office says Flynn assisted them 'on a range of issues, including interactions between individuals in the Presidential Transition Team and Russian Federation'.
Minutes later, Trump praised his 2016 presidential campaign adviser Roger Stone as someone who would "never testify against him", claiming that showed the former aide had "guts".
He is the only member of the Trump administration so far to plead guilty as a result of the investigation.
"Those who said the president's warm and complimentary tweet constitutes witness tampering should be reminded that I have never been contacted by any investigative body and therefore, by definition, this can not be true", Stone told the Daily Caller's Saagar Enjeti.
Friends and relatives have said Flynn, who at 2016 campaign rallies led chants of "Lock her up!" about Democratic nominee Hillary Clinton, is eager to face the judge and accept his fate following his dramatic guilty plea nearly exactly one year ago.
The special counsel pushed back Flynn's sentencing multiple times "due to the status" of the investigation.
Flynn also admitted that later in December 2016 he asked Kislyak not to retaliate in response to the Obama administration sanctions, something he initially told Federal Bureau of Investigation agents he didn't do.
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.
In September, the special counsel's office officially scheduled December 18 for Flynn's sentencing.
Flynn's case has been a contrast to those of other Trump associates, who have criticized the Russian Federation probe.
The Mueller-Cohen deal also involves cooperation with federal prosecutors.
He has instead spent considerable time with his family and worked to position himself for a post-conviction career. Flynn admitted that he lied about his conversations with then Russian ambassador Sergei Kislyak, and prosecutors have said that the two men had discussed sanctions that the United States had imposed on the Kremlin.
In this image made from a video taken on December 10, 2015 and made available on Tuesday, Feb. 14, 2017, US President Donald Trump's former National Security Advisor Michael Flynn, right, shakes hands with Russian President Vladimir Putin, in Moscow.
"The defendant provided firsthand information about the content and context of interactions between the transition team and Russian government officials", it noted. Flynn was under investigation by the Justice Department for the work when he became national security adviser.