The special counsel investigating alleged Russian interference in the 2016 US election will not be pursuing a jail sentence for Michael Flynn.
The former national security adviser to President Donald Trump has admitted lying to the FBI.
But in a memo, Robert Mueller said Flynn had provided “first-hand” details about links between the Trump election team and Russian officials.
The heavily redacted memo hints at future revelations that could trouble President Donald Trump.
President Trump has called the investigation a witch hunt and denies there was any collusion between his team and Russian officials to try to secure his election victory.
Mueller’s office released the memo as guidance to the judge who will sentence Michael Flynn, the only member of the Trump administration so far to plead guilty as a result of the collusion investigation.
He is due to be sentenced in the US District Court for the District of Columbia on 18 December.
Flynn has pleaded guilty to one count of making materially false statements to the FBI.
He admitted that he discussed lifting US sanctions on Russia with the Russian ambassador to Washington before Trump took office, and that he lied to the US vice-president about that conversation.