Former national security advisor Michael Flynn has given special counsel Robert Mueller "first-hand" details of contacts between President Donald Trump's transition team and Russian government officials, a bombshell court document filed Tuesday says.

Mueller in a sentencing memo said Flynn's "substantial assistance" to his probe warrants a light criminal sentence — which could include no jail time for the retired Army lieutenant general.

That assistance, which includes 19 interviews with Mueller's team and Justice Department attorneys, related to a previously unknown "criminal investigation," as well as to Mueller's long-running probe of the Trump campaign's and transition team's links or coordination with the Russian government.

"The defendant provided firsthand information about the content and context of interactions between the transition team and Russian government officials," the memo says.

Mueller's memo almost completely blacks out details of what Flynn might have said.

Trump's ex-national security advisor is due to be sentenced Dec. 18 in U.S. District Court in Washington.

He pleaded guilty last December to a single count of lying to federal agents about his conversations with Russia's ambassador to the United States during the presidential transition in late 2016.

Read Robert Mueller's sentencing memo for Michael Flynn

Flynn has cooperated with Mueller's ongoing probe since pleading guilty.

"Given the defendant's substantial assistance and other considerations set forth below, a sentence at the low end of the guideline range — including a sentence that does not impose a term of incarceration — is appropriate and warranted," Mueller's office wrote in the memo filed Tuesday.

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