Mueller cites Flynn's extensive cooperation in the Russian Federation probe and an unspecified criminal investigation, and his service in the military, for his recommendation to a District Court judge.
Last week, Trump's former attorney Michael Cohen pleaded guilty to lying to Congress about his work on the proposed Trump Tower Moscow during the 2016 campaign and prosecutors accused former Trump campaign chairman Paul Manafort of violating his cooperation agreement by lying to the special counsel.
Mueller wrote Tuesday that numerous investigations "in which [Flynn] has provided assistance are ongoing". Flynn made the request after discussing it with deputy national security adviser K.T. McFarland, who was at Trump's Mar-a-Lago resort, and being told that Trump's transition team did not want Russian Federation to escalate the situation.
So when it became apparent in the early days of Trump's presidency that Flynn was in the FBI's cross-hairs, and that both the agency and the highest levels of the Justice Department believed he had lied about his conversation with Kislyak, the news felt like a serious blow to the new president. The sheer amount of redacted text in the filing suggests those details are not ready to be unveiled, signaling that Mueller's investigation continues, along with other investigations that Flynn helped with.
Prosecutors said Flynn also provided help on other criminal investigations, although those details were blacked out to keep information about ongoing probes secret. It was filed two weeks ahead of Flynn's sentencing and just over a year after he pleaded guilty to lying to the Federal Bureau of Investigation about conversations with the Russian ambassador on Trump's behalf.
Prosecutors said the two men discussed US sanctions against Russian Federation and that Flynn also asked Kislyak to help delay a United Nations vote seen as damaging to Israel.
While the memo acknowledged Flynn's transgressions as "serious", it also noted that his assistance in the investigation was substantial enough to keep him from going to jail. He has also been scrutinized by the intelligence community, and likely Mueller, for his contacts with high-ranking Russians before and during his short time in the administration.
Flynn served as Trump's national security adviser for less than a month in early 2017, then was sacked more than two weeks after then-acting Attorney General Sally Yates told White House officials that Flynn lied to Vice President Mike Pence about his communications on Russian sanctions with then-Russian Ambassador to the US Sergey Kislyak.
Around that time, Trump asked then-FBI Director James Comey to let the Flynn investigation go, Comey has said.
Flynn's sentencing, which has been delayed a number of times for reasons that have never been disclosed, is scheduled to finally take place on December 18.
Mueller's office has had varied degrees of success with the level of cooperation it has received from defendants who have pleaded guilty. Two other defendants, Alex van der Zwaan and George Papadopoulos, served a 30-day sentence and a 14-day sentence, respectively.
He is so far the only member of Trump's administration to plead guilty to a crime uncovered during Mueller's wide-ranging investigation into Russian attempts to influence the 2016 USA election and potential collusion by Trump aides. The prosecutors said senior members of Trump's transition team publicly repeated false information conveyed to them by Flynn about his communications with Kislyak.
Third, because the addendum to the sentencing memo is mostly redacted, one is left reading tea leaves in the document's redactions. It is a spare document, seven pages, along with a largely-redacted six-page supplement laying out what Flynn has done for Mueller.
Flynn also admitted to making false statements about unregistered foreign agent work he performed for the benefit of the Turkish government. A highly redacted addendum to the memo said that Flynn had helped the government with several investigations, including the Russian Federation probe, and done 19 interviews with the special counsel's office or other Department of Justice offices.