Before that, Van Grack, told Sullivan that "it remains a possibility" that Flynn could continue to cooperate with Mueller's ongoing investigations even after being sentenced.
Van Grack noted that Flynn, in addition to other significant assistance to Mueller, had "provided substantial assistance" to federal prosecutors in Virginia, who on Monday revealed charges against former Flynn associates Bijan Kian and Kamil Alptekin.
They are charged with conspiring to smear and win the extradition of exiled cleric Fethullah Gulen at the behest and funding of the government of Turkey.
Sullivan began the hearing by pushing Flynn and his lawyers on their suggestion in a recent court filing that he might have been unfairly treated by the FBI in the case, which was lodged by Mueller.
Flynn told Sullivan that he did not want to withdraw his guilty plea or to challenge the fairness of the FBI interview that led to his plea.
Flynn's lawyer told the judge he had not been entrapped by the FBI.
"I was aware" that lying to the FBI was a crime, Flynn said, declining an offer by Sullivan to postpone the sentencing.
Some supporters of Flynn — along with Trump — had suggested he did not know that.
Defense lawyers last week in their court filing noted that Flynn did not have an attorney with during that Jan. 24, 2017, interview, and agents didn't warn him that he could be prosecuted if he lied to them.
"I cannot recall any incident in which the court accepted a guilty plea in which he was not guilty, and I don't intend to start today," Sullivan said.
"Are you continuing to accept responsibility for your false statements?" Sullivan asked.
"I am, your honor," Flynn answered.
Flynn lawyer Robert Kelner told Sullivan the reason the defense had talked about the circumstances of the FBI interview was because "we thought it was important for us to explain to the court those aggravating circumstances that are not present here today."
Kelner noted that a former Trump campaign advisor George Papadopoulos and other targets of Mueller had been warned of the risk of lying to investigators, and those defendants "did have counsel and lied anyway."
But Kelner also said of Flynn, "He fully accepts responsibility, stands by his guilty plea which was made based on knowing and willing conduct."
Sullivan said those cases were not analogous to Flynn's, since they did not involved high-ranking officials who committed crimes in the West Wing of the White House.
"This case is in a category by itself," Sullivan said.
Flynn's lawyer said, "General Flynn has held nothing back. Nothing."
David Weinstein, a former federal prosecutor who now does criminal defense work, told CNBC, "Given the judge's comments, it was not unheard of for Flynn's lawyers to seek a continuance of the sentencing hearing."
"While both the defense and the government were seeking a non-jail sentence, the court was not bound by that recommendation," Weinstein said. "By continuing the sentencing and his cooperation, the defense will be in a better position for the government to file a specific motion seeking a downward departure for his cooperation."
Since pleading guilty, Flynn has been cooperating with Mueller's ongoing investigation of Russian interference in the 2016 presidential election.
He has met more than 19 times with Mueller's team and other investigators. Mueller, in asking for leniency for Flynn, had cited Flynn's help in uncovering contacts between Russian government officials and members of Trump's presidential transition team.
Flynn's aborted sentencing came a week after Trump's former personal lawyer Michael Cohen was sentenced to three years in prison for financial crimes, and for campaign violations related to hush-money payments shortly before the election to two women who claim to have had affairs with Trump.
Cohen also had pleaded guilty to lying to Congress about details of an aborted plan by the Trump Organization to build a tower in Moscow.
Trump's former campaign chief Paul Manafort, a longtime Republican consultant, is scheduled to be sentenced for bank and tax fraud charges on Feb. 8.
Those crimes relate to Manafort's work in Ukraine on behalf of pro-Russia politicians before he joined Trump's campaign.
Flynn was interviewed by FBI agents just two days after being sworn in as national security advisor.
In that interview, according to court documents, he claimed that he did not recall ever asking then-Russia ambassador Kislyak during a phone call in December 2016 to not have Russia retaliate against the United States for sanctions imposed on the country after accusations that Russia had interfered in the recent presidential election.
Flynn also told the agents, "no," when asked if he made any comment to Kislyak about Russia voting in a certain manner at the United Nations on a resolution sponsored by Egypt condemning Israeli settlements in the West Bank.
Both of those statements to the agents were false.
Flynn resigned after just 24 days in office after he lied to Vice President Mike Pence about his conversations with Flynn.
Hours before Flynn was sentenced, Trump wished him "good luck" in a tweet that also again reiterated the president's claim that his campaign did not collude with Russians to influence the outcome of the 2016 election. Mueller is continuing to investigate that possibility.
Last year, a day after Flynn pleaded guilty, Trump had said in a tweet: "I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!"