Noting that he could not hide "my disgust, my disdain" for Flynn's conduct, District Court Judge Emmet Sullivan - who was appointed to various judicial posts by Ronald Reagan, George H.W. Bush and Bill Clinton - harshly criticized the former national security advisor, hinting that he might ignore Mueller's recommendation of a light sentence because of the aggravating circumstances.
The judge told Flynn he could not promise that he would not eventually sentence him to serve prison time.
Flynn, who faces up to six months in prison, has urged for leniency, saying he cooperated with special counsel prosecutors who are investigating Russian Federation and Trump's campaign. "The defendant chose to make false statements about his communications with the Russian ambassador weeks before the Federal Bureau of Investigation interview, when he lied about that topic to the media, the incoming Vice President, and other members of the Presidential Transition Team". Kelner said he expected Flynn would have to testify in a related trial in Virginia involving Flynn's former business associates, and the defence wanted to "eke out the last modicum of co-operation" so he could get credit.
"I am going to be frank with you, this crime is very serious", the judge said.
Revelations of those discussions led to Mueller's probe of his actions. He met privately with investigators 19 times and provided cooperation so extensive that prosecutors said he was entitled to avoid prison altogether.
He later softened his tone, apologizing for suggesting that Flynn had worked as a foreign agent while in the White House when that other work had actually already ended.
Flynn's legal woes stem from transition-period calls with then-Russian Ambassador Sergey Kislyak that raised intelligence community alarms even before Trump took office. "The only problem is that Robert Mueller's team feels that he has been done with cooperation, and it is not clear how much cooperating he could do to basically wash away the sins here, and come away with no prison time".
In a series of biting questions, Judge Sullivan undermined Flynn's lawyers' line of argument, pressing Flynn on whether he knew that lying to the Federal Bureau of Investigation was a crime and pushing his attorneys to make the case that their client had indeed been entrapped.
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During a hearing at a Washington, D.C. courthouse, federal Judge Emmet Sullivan made a decision to postpone the sentencing date until Flynn completes his cooperation with prosecutors.
Flynn has not tried to retract his guilty plea, and there's every indication the sentencing will proceed as scheduled. Court papers show Flynn made the request at the direction of Trump son-in-law Jared Kushner.
A group of protesters, including some who chanted "Lock him up", gathered outside the courthouse on Tuesday, along with a large inflatable rat fashioned to look like Trump.
Mr Comey has said he believes the president was trying to shut down the inquiry into Flynn's unlawful contacts with a foreign government.
He became national security adviser when Trump took office in January 2017 but lasted only 24 days before being ousted.
"We wish General Flynn well", Sanders said.
But those plans seemingly went awry after Sullivan lambasted Flynn for lying to the Federal Bureau of Investigation and for working as an unregistered foreign agent of Turkey.