The filing, submitted by prosecutors to show the extent of Flynn's cooperation with the Russian Federation probe, did not identify the people who contacted Flynn or describe specific conversations. "And you would have thought it would have, and they would have maybe focused on that, but it didn't come up at all", said Flynn's brother.
Mueller's final report had made clear Flynn had contributed to Mueller's probe into the transition's communication with then-Russian ambassador Sergey Kislyak and possible obstruction of justice by the President.
Prosecutors said Flynn "provided a voicemail recording" related to "communications from persons connected to the administration or congress that could have affected both his willingness to cooperate and the completeness of that cooperation".
Flynn later became a key figure in special counsel Robert Mueller's investigation into Russian interference in the 2016 election.
Flynn pleaded guilty in December 2017 to a charge of lying to the Federal Bureau of Investigation about his communications with Russian ambassador Sergey Kislyak in the weeks before Trump took office. It could reveal how others tried to obstruct Mueller's investigation.
Flynn, a former national security advisor, not only told investigators about these communications but also provided the special counsel's office with a voicemail of one instance.
No other details were provided in the filing, but the Mueller report noted that President Donald Trump's personal lawyer left a voicemail message for Flynn in late November 2017 that addressed the possibility of him cooperating with the government.
The 400-page Mueller report concluded in March there was insufficient evidence to show collusion between the campaign and Russian Federation, and raised questions as to whether Trump obstructed justice by trying to derail the Justice Department investigation. His forced resignation occured after information surfaced about how he misled the Federal Bureau of Investigation and Vice President Mike Pence about his communication with Russian Ambassador to the U.S. Sergey Kislyak.
Flynn had been scheduled to be sentenced on December 18 but the judge fiercely criticised the retired US Army lieutenant general for his actions and delayed his sentencing until after he has finished helping prosecutors with other probes.
Flynn was supposed to have been sentenced in December, with prosecutors saying he was so cooperative and helpful in their investigation that he was entitled to avoid prison. "For example, the defendant recalled conversations with senior campaign officials after the release of the Podesta emails, during which the prospect of reaching out to WikiLeaks was discussed", according to the newly unredacted documents.
The document also details how Flynn assisted investigators as they looked into whether the Trump campaign conspired with the Kremlin to sway the outcome of the 2016 election.