WASHINGTON — Federal prosecutors suggested on Tuesday that they might ask a federal judge to sentence the former national security adviser Michael T. Flynn to prison as doubts have emerged about whether he honored the cooperation deal he made in his criminal case.
The move was the latest turn in the case of Mr. Flynn, who pleaded guilty in 2017 to lying to F.B.I. agents in the Russia investigation and agreed to cooperate with prosecutors to help make the case that he deserved a light sentence.
But in recent months, Mr. Flynn has taken a more aggressive posture. He has hired new lawyers, who accused the government on Tuesday of hiding exculpatory evidence, as they have in court papers.
Judge Emmet G. Sullivan of the Federal District Court in the District of Columbia asked one of the prosecutors, Brandon L. Van Grack, whether the government stood by its recommendation that Mr. Flynn deserved no prison time. Mr. Van Grack said he intended to refile sentencing paperwork, an assertion that suggested the government had possibly changed its stance.
The judge asked Mr. Flynn’s lawyer, Sidney Powell, who has accused the government of “egregious” misconduct, whether she wanted to scrap the plea agreement, but she held off on committing to such a drastic step. “I can’t say right now exactly where it’s headed, but I don’t think it’s going to be a motion to withdraw the plea,” she said of her defense, adding that the “entire prosecution should be dismissed.”
Mr. Flynn admitted to lying to investigators in early 2017 about his communications with Sergey I. Kislyak, the longtime Russian ambassador to the United States. Prosecutors said Mr. Flynn’s “false statements and omissions impeded and otherwise had a material impact” on the Russia investigation. Mr. Flynn faced up to six months in prison for lying to federal law enforcement officials.
Prison time would signal a downfall for Mr. Flynn, a retired general who once led the Defense Intelligence Agency before he was forced out in 2014. Mr. Flynn was a top backer of Mr. Trump’s campaign, infamously leading supporters in chants of “lock her up” against Hillary Clinton at the Republican National Convention in 2016.
As part of his plea deal to avoid jail time, Mr. Flynn cooperated with prosecutors, answering questions in the Russia inquiry and investigators in Virginia with a case against his former business partner, Bijan Kian.
Though the case always carried a possibility of prison time, his significant cooperation would be an important factor in sentencing. Mr. Flynn had even asked Judge Sullivan last year to delay his sentencing until he testified against Mr. Kian.
But instead of testifying in that case, prosecutors decided not use Mr. Flynn as a witness because he changed his account on the eve of the trial. Mr. Kian was later convicted on violating foreign lobbying laws, but a hearing is set for Thursday about whether a conspiracy conviction should be thrown out because of a lack of evidence.
Mr. Flynn’s lawyers have insisted he cooperated fully with prosecutors, consuming his time and money.
On Tuesday, Ms. Powell again accused prosecutors of hiding exculpatory evidence, known as Brady material, that would exonerate her client. Ms. Powell is known for her dislike of the special counsel and is a staunch supporter of President Trump.
It was not clear why Ms. Powell told the judge she did not want to vacate the plea deal if she believes the government engaged in “stunning failures” to produce the exculpatory material. Criminal lawyers observing the case say that would be a logical move if she were convinced that the government had committed serious wrongdoing and she could prove it.
Prosecutors pushed back against some of her claims. Ms. Powell said that text messages between former F.B.I. officials disparaging Mr. Trump — including from one of the agents who questioned Mr. Flynn — would have helped her client’s case by showing bias among investigators.
But prosecutors said Mr. Flynn’s previous lawyers were given the texts on two occasions before he pleaded guilty. And while Ms. Powell said a Justice Department inspector general report “documented” the bias, the report explicitly said that internal investigators found no evidence of bias that affected investigative decisions. The report did say, however, that the texts “cast a cloud” over the F.B.I.’s handling of the Hillary Clinton email investigation.
In addition, Ms. Powell said that she wanted to see a Justice Department memo that she described as exonerating Mr. Flynn of being an agent of Russia. The government has not accused Mr. Flynn of that role.
“That is not part of this case,” Mr. Van Grack said. “That is not part of the act that he has pled guilty to. It was lying to the F.B.I. about particular communications involving Russia, not being an agent of Russia.”
Ms. Powell has asked Judge Sullivan to compel prosecutors to turn over Brady material and said she needs a security clearance to review classified information. Prosecutors say the case did not involve classified information. Ms. Powell told the judge that there would have been no need for a plea deal if the government had honored its Brady obligations.
The judge is expected to decide those matters in the coming weeks and added that he wanted to bring some finality to the case, setting a tentative sentencing date for Dec. 18.