Former U.S. National Security Adviser Michael Flynn departs after a plea hearing at U.S. District Court, in Washington, U.S., December 1, 2017.

Special counsel Robert Mueller and defense lawyers for former national security advisor Michael Flynn on Monday asked that the retired Army lieutenant general be sentenced for his crime of lying to the FBI on Nov. 28 or the following week.

The request suggests that Flynn has completed, or soon is expected to complete, his cooperation with Mueller in the prosecutor's ongoing probe of Russian interference in the 2016 presidential election, as well as into possible collusion by President Donald Trump's campaign in that effort, and other matters.

Mueller himself was appointed special counsel after Trump fired FBI Director James Comey in May 2017, two months after Comey said the president had said, "I hope you can let this go," referring to an ongoing criminal probe of Flynn.

The former national security adviser pleaded guilty last December to lying to the FBI about the nature of his conversations with Russia's then-ambassador to the United States, Sergey Kislyak, which occurred after the election, but before Trump was sworn in as president.

Flynn had talked to Kislyak about sanctions the Obama administration was imposing on Russia for meddling in the election, but later claimed falsely that the talks were not substantive.