WASHINGTON — Lawyers for Michael T. Flynn, President Trump’s first national security adviser, asked a federal judge late on Tuesday to spare their client any prison time for pleading guilty to lying to investigators, citing his significant cooperation with the special counsel’s investigation.
Mr. Flynn’s lawyers said his lengthy military service and willingness to assist the special counsel, Robert S. Mueller III, should warrant a sentence of only probation. Lawyers also included letters from supporters vouching for Mr. Flynn’s character.
“His cooperation was not grudging or delayed,” Mr. Flynn’s lawyers wrote in a sentencing memorandum.
Mr. Flynn faces up to six months in prison when he is sentenced on Dec. 18, but a punishment of that length seems unlikely. Federal prosecutors also told Judge Emmet G. Sullivan of the United States District Court for the District of Columbia last week that Mr. Flynn, a retired Army lieutenant general, deserved little to no prison time. They cited the substantial information he provided in several ongoing federal inquires, including sitting for 19 interviews with Mr. Mueller’s team and other investigators.
Mr. Flynn also “deserves credit for accepting responsibility in a timely fashion and substantially assisting the government,” prosecutors wrote. His decision to plead guilty and cooperate, they wrote, “likely affected the decisions of related firsthand witnesses to be forthcoming.”
Mr. Flynn’s cooperation gave the special counsel, who is investigating whether Trump associates conspired with Russia’s 2016 election interference, a source of information about how Mr. Trump and his advisers responded in the months after his surprise election victory and from inside the Oval Office during the administration’s chaotic first weeks.
Mr. Flynn pleaded guilty late last year to lying to the F.B.I. about conversations he had during the transition in late 2016 with the Russian ambassador to the United States at the time, Sergey I. Kislyak.
They spoke about sanctions that the Obama administration had imposed that day on Russia over its interference in the election, and Mr. Flynn asked that Russia temper its response. The exchange flouted a warning from a senior Obama administration official to stop meddling in foreign affairs before Mr. Trump took office.
Mr. Flynn and Mr. Kislyak also separately discussed a coming United Nations Security Council vote on whether to condemn Israel’s building of settlements. Prime Minister Benjamin Netanyahu of Israel asked the Trump transition team to press other countries to help Israel, prosecutors have said.
Agents interviewed Mr. Flynn a week after Mr. Trump took office, and he denied asking Mr. Kislyak that Russia refrain from reacting harshly to the sanctions and said he did not remember Mr. Kisklyak saying that Moscow had backed off as a result of Mr. Flynn’s request.
Because Mr. Flynn had been untruthful with investigators, senior law enforcement officials warned the White House that he could be at risk of being blackmailed by Russia. Mr. Trump and his aides reviewed the situation and concluded that Mr. Flynn had no legal exposure. But he was fired after only 24 days when the conversations became public, and aides said he had misled Vice President Mike Pence about the calls.
In his plea agreement, prosecutors said Mr. Flynn’s lies and omissions hurt the Russia investigation.
Mr. Flynn had a decorated career in the Army, rising to become head of the Defense Intelligence Agency before President Barack Obama fired him in 2014 over management failures.
Mr. Flynn went on to create his own consulting company. Among his clients was the government of Turkey, which paid him more than a half-million dollars to investigate Fethullah Gulen, a Turkish cleric who lives in Pennsylvania.
President Recep Tayyip Erdogan of Turkey has held Mr. Gulen and his supporters responsible for a failed coup attempt in 2016 and has repeatedly demanded that the United States extradite Mr. Gulen, who fled Turkey in 1999.
Mr. Flynn has admitted that he failed to properly register as an agent of Turkey, and federal prosecutors in Virginia are investigating the secret Turkish lobbying effort that once ensnared him.
Mr. Trump has said Mr. Flynn had to leave the White House after deceiving Mr. Pence, but the president has defended his former national security adviser, saying investigators treated him unfairly.
Mr. Flynn has also figured into the special counsel’s investigation into whether the president obstructed justice. James B. Comey, the former F.B.I. director, has told lawmakers that the president asked him to shut down the F.B.I. investigation into Mr. Flynn.
Asked last week about Mr. Flynn’s case, Mr. Comey said he was glad Mr. Flynn was “held accountable for his crimes and that he was assisting the United States. So it seemed to me like a just outcome.”
Mr. Comey also sought to clarify earlier testimony about the F.B.I.’s interview with Mr. Flynn, which some conservatives have cited as potential evidence that Mr. Flynn did not lie to investigators. Mr. Comey dismissed that theory.
“The conclusion of the investigators was he was obviously lying,” Mr. Comey said, “but they saw none of the normal common indicia of deception: that is, hesitancy to answer, shifting in seat, sweating, all the things that you might associate with someone who is conscious and manifesting that they are being — they’re telling falsehoods.”