WASHINGTON — A federal judge on Tuesday postponed the sentencing of Michael T. Flynn, President Trump’s first national security adviser, after warning Mr. Flynn that he could face prison for lying to federal investigators about his conversations with the Russian ambassador during the presidential transition and hiding his role lobbying for Turkey.
At Mr. Flynn’s sentencing hearing in Federal District Court in Washington, Judge Emmet G. Sullivan called Mr. Flynn’s crimes “a very serious offense” and said he was not hiding his “disgust” at what Mr. Flynn had done.
“All along you were an unregistered agent of a foreign country while serving as the national security adviser,” the judge told Mr. Flynn. “Arguably that undermines everything that this flag over here stands for. Arguably you sold your country out.”
Later in the hearing, the judge corrected himself, noting that Mr. Flynn’s work on behalf of Turkey had ended in mid-November of 2016, before Mr. Flynn became national security adviser. The judge acknowledged he had made a mistake and said he felt “terrible about that.”
But Judge Sullivan gave Mr. Flynn the option of delaying the sentencing until he had completed his cooperation agreement with federal prosecutors. “I cannot assure that if you proceed today you will not receive a sentence of incarceration,” Judge Sullivan told Mr. Flynn.
After a short recess, Mr. Flynn returned to the courtroom to take the judge up on his offer.
Mr. Flynn faces up to six months in prison, but federal prosecutors have recommended a lenient sentence, including the possibility of probation, because Mr. Flynn has provided “substantial help” with multiple criminal inquiries.
During the sentencing hearing, Judge Sullivan questioned Mr. Flynn and his lawyer about their earlier suggestion that F.B.I. agents might have tricked Mr. Flynn by failing to inform him before they interviewed him nearly two years ago that lying to them would constitute a federal crime.
Mr. Flynn told the court that he was not challenging the circumstances of the interview and that he knew lying to the F.B.I. was a crime. In doing so, Mr. Flynn distanced himself from Mr. Trump’s efforts to suggest misconduct by the F.B.I. in the investigation by the special counsel, Robert S. Mueller III.
Earlier, Mr. Trump had wished Mr. Flynn “good luck” in a Twitter post.
Mr. Flynn is the highest-ranking aide to Mr. Trump to face sentencing in the special counsel’s investigation of Russia’s interference in the 2016 presidential election and the Trump campaign. His case has marked an extraordinary fall from grace for a retired three-star general who once headed one of the nation’s most important military intelligence operations, the Defense Intelligence Agency.
Prosecutors have refused to disclose publicly the details of how Mr. Flynn, 59, helped them during 19 interviews over the past year, redacting paragraph after paragraph of their sentencing memo to the judge. His lawyer, Robert K. Kelner, said in court on Tuesday that Mr. Flynn’s cooperation was “very largely complete” but that Mr. Flynn wanted to make sure he got full credit for further assistance to prosecutors before being sentenced.
Judge Sullivan made abundantly clear throughout the proceedings that he viewed the crimes admitted to by Mr. Flynn as extraordinarily serious and a betrayal of the trust placed in him as a high-ranking White House official. At one point he even asked prosecutors if Mr. Flynn might have committed treason. (The prosecutor in the case, Brandon L. Van Grack, said no.)
The special counsel’s office is investigating whether Mr. Trump obstructed justice, including by asking James B. Comey, the F.B.I. director at the time, to end the investigation of Mr. Flynn in early 2017. It is unclear whether Mr. Flynn knew about the president’s reported attempt to intervene on his behalf.
On Monday, federal prosecutors in Virginia unsealed an indictment accusing two of Mr. Flynn’s former business associates of violating foreign lobbying rules. Prosecutors said the two men conspired with Turkey in 2016 to pressure the United States to expel a rival of President Recep Tayyip Erdogan. Prosecutors said Tuesday that Mr. Flynn aided them in the case.
In arguing for probation, Mr. Flynn’s lawyers had cited his lengthy military service, his cooperation with prosecutors and his contrition.
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But they also had criticized F.B.I. agents for failing to advise him before the interview on Jan. 24, 2017, that lying to them would constitute a federal crime. They claimed that the agents deliberately did not warn Mr. Flynn so he would not be on his guard — an accusation that appeared intended to draw the attention of Judge Sullivan, who has taken other prosecutors to task for misconduct.
Defense lawyers also raised the idea that Mr. Flynn’s bearing during questioning was potential evidence that he did not lie to investigators. One of the agents who interviewed Mr. Flynn later told the special counsel that Mr. Flynn had a very sure demeanor and did not reveal any “indicators of deception.”
The move by Mr. Flynn’s legal team to raise questions about the F.B.I.’s conduct might have been a play for a pardon from the president, whose former lawyer had discussed the idea last year with a lawyer for Mr. Flynn. Mr. Trump has repeatedly said that Mr. Flynn was treated poorly.
Prosecutors dismissed the claims that Mr. Flynn had been tricked as a poor excuse, saying that as a high-ranking White House official and the former director of an intelligence agency, he was well aware that misleading federal authorities was a felony offense.
“The seriousness of the defendant’s offense cannot be called into question, and the court should reject his attempt to minimize it,” prosecutors wrote last week after Mr. Flynn’s legal team made the assertion. In an account of Mr. Flynn’s F.B.I. interview filed in court late Monday, agents described in detail how he falsely answered their questions.
His sentencing hearing came amid a flurry of activity in criminal cases that have involved the Trump campaign, the White House and the president himself.
Last week, Michael D. Cohen, Mr. Trump’s longtime fixer, was sentenced to three years in prison for crimes including organizing hundreds of thousands of dollars in payments to cover up potential sex scandals that threatened Mr. Trump’s presidential bid. Prosecutors have said Mr. Cohen acted at Mr. Trump’s direction, implicating the president in felony violations of campaign finance laws.
Mr. Flynn, who led Trump supporters in chants of “lock her up” against Hillary Clinton at campaign rallies, was interviewed by F.B.I. agents only four days after Mr. Trump’s inauguration. He pleaded guilty a year ago to misleading them about a series of discussions he had with the Russian ambassador, Sergey I. Kislyak.
Prosecutors have said Mr. Flynn’s deceptions impeded the F.B.I.’s open investigation into possible links between the Trump campaign and Moscow’s covert effort to tip the presidential election in Mr. Trump’s favor. Mr. Trump has said he fired Mr. Flynn because he had also lied to Vice President Mike Pence about his conversations with the Russian ambassador.
Mr. Flynn has now admitted that after the outgoing Obama administration imposed sanctions against Russia for its interference in the 2016 presidential race, he requested that Russia not escalate tensions between the two countries. Mr. Kislyak later told him that Russia had agreed not to retaliate, an unusual decision that Mr. Trump himself praised.
But in his interview with the F.B.I., Mr. Flynn claimed that he did not remember ever asking Mr. Kislyak that Russia hold back, according to the agents. He told them that he did not even know about the Obama administration’s decision to expel dozens of Russian diplomats and to seize two Russian-owned estates because at that time he was on vacation in the Dominican Republic, without access to television or to his government-issued BlackBerry phone.
Mr. Flynn has also acknowledged that he lied to the F.B.I. about his discussions with Mr. Kislyak and officials from other countries about an impending vote on a United Nations resolution condemning Israeli settlements in the West Bank.
The agents said Mr. Flynn told them that he asked Mr. Kislyak about Russia’s views but did not advocate Russia take any particular position on the resolution. He “stated the conversations were along the lines of where do you stand, and what’s your position,” the agents wrote.
In fact, Mr. Flynn asked that Russia either delay or oppose the resolution.
Finally, he has admitted lying about his lobbying work for Turkey in documents he filed with the Justice Department after he was forced out as Mr. Trump’s national security adviser amid controversy over his conversations with the Russian ambassador.
He held that post for just 24 days, the shortest tenure ever.