On May 16, 2019, the U.S. District Court for the District of Columbia unsealed sentencing documents for Michael Flynn. Included in those documents were descriptions of how Flynn assisted prosecutors, including both the cases he had been involved with, and the way in which his cooperation had assisted the investigation.
From the Daily Kos: “The documents show that Flynn’s cooperation was vital in three different cases: the criminal investigation into how Flynn’s former business partner acted as an unregistered agent for Turkey; the special counsel investigation into connections between the Trump campaign and Russia; and a third case, the description of which remains redacted. Some speculation has suggested that the third case may be related to the ongoing trial of Roger Stone, or other potential cases related to WikiLeaks. Others have suggested that it might represent Flynn’s knowledge of some financial matter related to the Trump Organization … which mostly shows just how little is known about the multiple cases still pending in various jurisdictions following the Mueller investigation.”