Michael Flynn, President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE’s first national security adviser who began cooperating with special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s investigation about a year ago, will be sentenced Tuesday for lying to the FBI.

Flynn's appearance in D.C. federal court before U.S. District Judge Emmet Sullivan will be viewed as a key milestone in an investigation that has plodded along for 19 months amidst high public intrigue and increasing vitriol from the president.

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It is arguably the most highly anticipated sentencing yet in Mueller's investigation and comes on the heels of the three-year sentence handed down to Trump’s former personal attorney Michael Cohen for a slew of federal charges that sprung from details uncovered by Mueller’s sprawling probe.

Flynn is unlikely to be sentenced to significant prison time. Citing his “substantial assistance” in ongoing investigations, Mueller recommended a lenient sentence for Flynn and has not asked for any jail time.

But Flynn and his attorneys may have scrambled his situation by arguing that Flynn was entrapped by the FBI into lying about his Russia contacts, giving new life to a theory among conservatives that the onetime Trump campaign surrogate was wronged by the Justice Department and Mueller.

Flynn’s defense attorneys asked the court to spare him from prison and sentence him to at most one-year probation last week. They cited an FBI report stating that agents did not warn Flynn of the penalties for making a false statement “because they wanted Flynn to be relaxed, and they were concerned that giving the warnings might adversely affect the rapport.”

Without disputing Mueller’s characterization of the crime, they argued Sullivan should consider the circumstances of his interview when evaluating “the seriousness of the offense.”

The court filing added fresh drama to Flynn’s case and prompted Sullivan, who has a reputation as a hawk for any government misconduct, to ask Mueller to produce documents related to Flynn’s interview, which the special counsel filed in redacted form on Friday and Monday.

It also inspired a firm rebuke from Mueller, who wrote that Flynn “chose” to lie and asserted that Flynn, a retired three-star Army general and former chief of the Defense Intelligence Agency, “knows he should not lie to federal agents.”

Mueller also asked Sullivan to reject Flynn’s attempt to “minimize the seriousness of those false statements to the FBI.”

The developments have energized conservatives who have speculated that Flynn was mistreated by the FBI and duped into lying. One of the agents who interviewed Flynn was former agent Peter Strzok, who became a popular target among Republicans after a Justice Department inspector general probe unearthed text messages he sent critical of Trump prior to the 2016 presidential election.

Trump himself suggested last week that Mueller’s prosecutors gave Flynn a “great deal because they were embarrassed by the way he was treated” and attempted to “scare” him into making up stories. The president has also seized on accounts that agents did not observe outward signs indicating Flynn knew he was lying as evidence his former national security adviser was wrongly ensnared Mueller’s probe.

Some legal experts say that Flynn's decision to cast his FBI interview as deceptive could backfire, if the judge views him as downplaying his culpability.

“I think it’s kind of surprising, and actually potentially risky on his part,” said Randall Eliason, a George Washington University law professor and former assistant U.S. attorney in D.C. “That could actually work against him.”

Federal sentencing guidelines call for Flynn to face between zero and six months in prison and up to a $9,500 fine. Others ensnared in Mueller’s probe who have pleaded guilty to the same offense received small sentences, including George PapadopoulosGeorge Demetrios PapadopoulosTale of two FBI cases: Clinton got warned, Trump got investigated Trump says he would consider pardons for those implicated in Mueller investigation New FBI document confirms the Trump campaign was investigated without justification MORE, a former Trump campaign adviser who was released from prison after 12 days.

Flynn, once a vocal Trump campaign surrogate, served less than a month in the White House before he was forced to resign over revelations he misled Vice President Pence and other administration officials about his conversations with then-Russian Ambassador Sergey Kislyak regarding sanctions on Moscow during the transition.

Last December, Flynn admitted to also lying to FBI agents about his contacts with Kislyak, pleading guilty to a single false statement charge as part of a deal to cooperate in Mueller’s probe.

Flynn is viewed as a valuable witness for Mueller as he probes whether the Trump campaign coordinated with Moscow to interfere in the election. Mueller revealed earlier this month that Flynn had provided details on the “content and context” of interactions between unnamed members of the transition team and the Russian government, among other unknown subjects.

It is unclear whether or to what extent Flynn’s cooperation with Mueller poses a threat to Trump or any of his associates. Flynn is said to have provided “firsthand” insights from his work on the campaign and could be a key witness for the government if it has uncovered criminal conduct and pursues new charges. Trump insists there was no collusion between his campaign and Moscow and views the probe as a partisan “witch hunt.”

It is possible but unlikely that Tuesday’s hearing will shed new light on the information Flynn has given to government prosecutors as it relates to the core Russia probe or other criminal investigations. Flynn took part in 19 interviews with Mueller’s prosecutors and other Justice Department officials.

On the eve of his sentencing, federal prosecutors in Virginia unsealed charges against his former business partner for illegally lobbying on behalf of the Turkish government.

There is no clear end in sight for Mueller’s investigation, though there is considerable speculation that the probe is approaching its late stages. Trump has provided written answers to Mueller in response to questions of collusion but has resisted an in-person interview.

Trump’s personal attorney Rudy Giuliani said on “Fox News Sunday” that only would happen over his “dead body.”

“After what they did to Flynn, the way they trapped him into perjury, and no sentence for him,” Giuliani said. “Over my dead body.”