US District Judge Emmet Sullivan ruled to delay the former national security adviser Michael Flynn's sentencing until March following a tense hearing on Tuesday.

"Arguably," Sullivan told Flynn, "you sold your country out."

Flynn pleaded guilty last December to one count of lying to the FBI as part of the bureau's Russia investigation.

He has been cooperating with investigators for nearly a year, and prosecutors said it was possible he would cooperate more.

Sullivan said that "this crime" Flynn committed "is very serious" and that he would not hide his "disdain" for Flynn's actions.

"In the White House! In the West Wing!" Sullivan said, adding that Flynn "can't minimize" his "very serious offense."

US District Judge Emmet Sullivan agreed to delay the sentencing of the former national security adviser Michael Flynn on Tuesday following criticism from Sullivan and a request from Flynn's defense attorney.

Flynn has been cooperating with federal investigators for nearly a year, and prosecutors hinted on Tuesday that he could cooperate more.

Flynn pleaded guilty last December to one count of lying to the FBI about his communications during the presidential transition period with Sergey Kislyak, then Russia's ambassador to the US. Flynn also admitted to lying about the lobbying work he did for the Turkish government.

President Donald Trump has frequently suggested that Flynn was duped into lying to the FBI. On Tuesday, Flynn threw cold water on that assertion and said he was aware that lying to the FBI was a crime when he misled investigators.

During the sentencing hearing, Sullivan asked Flynn whether he wanted to challenge the circumstances under which he was interviewed by the FBI.

"No, your honor," Flynn replied.

Sullivan later gave Flynn another chance to withdraw his guilty plea if he felt he had been unfairly coaxed into it.

"I would like to proceed," Flynn said.

Asked whether he wished to proceed because he was guilty, Flynn said, "Yes, your honor."

Sullivan asked prosecutors whether Flynn was still cooperating.

They said it "remains a possibility," adding that they decided to move forward with his sentencing because he had given them the "vast majority" of potentially useful information. They emphasized an indictment unsealed on Monday against his former associate, who was charged with acting as an illegal agent of the Turkish government.

Nonetheless, Sullivan said, "this crime" that Flynn committed, lying to the FBI, "is very serious."

"In the White House! In the West Wing!" Sullivan said, adding that Flynn "can't minimize" his "very serious offense."

"Arguably," Sullivan said, "you sold your country out."

Sullivan also asked the government whether Flynn could be charged under the Logan Act, and even whether he could face charges of treason. Prosecutors declined to broach that territory.

"I'm not hiding my disgust, my disdain for this criminal offense," Sullivan said.

After the court returned from its recess a little after noon on Tuesday, Sullivan walked back some of his comments, saying he was merely asking about treason, not implying that Flynn committed it.

"Don't read too much into the questions I asked," he said.

Robert Mueller. Alex Wong/Getty Images

What Flynn lied about

According to a memo released by the special counsel Robert Mueller's office late Monday, Flynn portrayed his conversations with Kislyak as more benign than they were during a January 2017 interview with FBI agents.

The memo said Flynn told investigators he had not tried to influence Russia's vote on a United Nations Security Council resolution about Israeli settlements. But investigators knew Flynn had asked Russia to vote against or delay the resolution, according to charging documents filed in his case last year.

Mueller's memo said Flynn also lied when FBI agents questioned him about whether he had asked Kislyak during a phone call in December not to retaliate following President Barack Obama's expulsion of Russian diplomats and closing of Russian diplomatic facilities in response to the Kremlin's meddling in the 2016 US election.

"Not really," Flynn replied, according to the memo. "I don't remember. It wasn't, 'Don't do anything.'"

But his answers contradicted signals intelligence the FBI had collected that indicated Flynn had asked Kislyak during the phone call not to punch back.

Read more: Mueller sent a clear message — and a warning — with Flynn's sentencing memo

In a sentencing memo Mueller's office filed in Flynn's case last week, prosecutors recommended Flynn get little to no jail time, citing the "substantial assistance" he provided to not only the special counsel's investigation into Russia's interference in the 2016 election, but separate, ongoing investigations.

Flynn was among the first defendants to plead guilty in the Russia investigation. His sentencing was delayed five times after prosecutors said they needed more information from the former national security adviser, who was a key surrogate for Trump's 2016 campaign.

Flynn emerges as a valuable asset

President Donald Trump and Flynn. George Frey/Getty Images

In their sentencing memo, prosecutors emphasized that Flynn began cooperating early, that his assistance was critical in encouraging other witnesses to come forward and be candid, and that he helped the investigation in several ways.

In typical fashion, Mueller's office revealed very little information, if any, about the specifics of what Flynn said. Prosecutors also attached a mostly redacted six-page addendum to the sentencing memo laying out how Flynn had cooperated in what appears to be a separate investigation not being conducted by Mueller's office.

In the sentencing memo, prosecutors laid out the crux of what Flynn was charged with: lying to the FBI about conversations he had with Kislyak about US sanctions on Russia.

Read more: Here's a glimpse at Trump's decades-long history of business ties to Russia

But that's not the only thread in which Flynn may have had valuable information.

The former national security adviser was also not forthcoming about his involvement in three key areas: his now defunct lobbying firm's work for the Turkish government in 2016, his efforts with Russia and Saudi Arabia to develop nuclear reactors in the Middle East — a project that would have benefited from the US lifting sanctions on Russia — and payments he received from the state-owned media outlet Russia Today that he failed to disclose when he applied to renew his security clearance in January 2016.

Flynn was also a participant in a controversial meeting during the transition period with Kislyak and Jared Kushner, a senior adviser to Trump, in which Kushner is said to have discussed setting up a secret back channel between the Trump team and Moscow using Russian diplomatic facilities.

And Flynn was an adviser to the campaign in June 2016 when Kushner, Donald Trump Jr., and Paul Manafort, then the campaign chairman, met in Trump Tower with two Russian lobbyists offering dirt on Hillary Clinton's campaign. The meeting is a focus of investigators for the special counsel and congressional committees.

'Flynn gave up the goods'

Jared Kushner. REUTERS/Lucas Jackson

Flynn's sentencing memo said he provided "firsthand information" about contacts between Russian officials and members of the Trump transition team.

"We do not yet know everything about Flynn's cooperation," Elie Honig, a former federal prosecutor from the Southern District of New York, told INSIDER in an interview. "The heavy redactions in the filing hide nearly all the details that Mueller provided to the court."

Jens David Ohlin, a vice dean at Cornell Law School who's an expert on criminal law, echoed that view.

"There is more to come, but we don't know what that is," he told INSIDER. The substantial redactions in Flynn's sentencing memo indicate that the focuses of other investigations he's cooperated with have not yet been charged, he said.

"It could be a reference to an obstruction-of-justice investigation, possibly involving the president, but we don't really know," Ohlin said. "So this is a known unknown."

Honig, however, emphasized that "if you read around all that black ink, Mueller did say enough to let us know this: Flynn gave up the goods, and Mueller's work is far from over."