A lawyer for Michael Flynn accused federal prosecutors Tuesday of withholding documents that would “completely exonerate” President Trump’s former national security adviser.

Sidney Powell leveled the bombshell accusation in federal court, nearly two years after Flynn admitted he lied to FBI investigators about his contact with the former Russian ambassador in 2016.

Ms. Powell’s claims signaled Flynn’s new strategy of attacking special counsel Robert Mueller’s Russia investigation, a strong contrast from Flynn’s previous legal approach of painting himself as an eager cooperator willing to work with prosecutors.

The new game plan may put Flynn’s freedom at risk. Last year, prosecutors said they would spare Flynn any jail time, citing his extensive cooperation. But on Tuesday, Justice Department attorney Brandon Van Grack said the government intended to file a new sentencing memo for Flynn, suggesting the government may reverse course from its leniency recommendation.

Even as Judge Emmet G. Sullivan set a tentative date of Dec. 18 for Flynn to bring “finality” to the case, Ms. Powell argued there were too many unresolved issues.

“There is far more at stake here than sentencing,” she told the court. “As new counsel, we have an ethical obligation to review everything that has happened or has not happened as the case may be.”

Although Ms. Powell said she was not asking to have Flynn’s guilty plea tossed, she claimed “the entire prosecution should be dismissed” because of “egregious” government misconduct and suppression of evidence.

Despite numerous claims the government is withholding evidence, Ms. Powell did not disclose Tuesday which documents she is seeking or what she thinks they will reveal. She did cryptically claim there is a January 2017 internal Justice Department memo that “exonerates” Flynn of being a Russian agent.

Mr. Van Grack countered that Flynn pleaded guilty to lying about his communications with the Russian ambassador, not being a Russian agent.

Ms. Powell continued, ticking off a vague list of material she claimed could prove Flynn’s innocence, including an unnamed document in the FBI’s computer system and a polygraph test.

“There is just a plethora of information out there that we need the actual documents to support and the notes to support,” she said.

Mr. Van Grack emphatically rejected Ms. Powell’s claim of legal shenanigans. He told the court the government “exceeded” its discovery obligations, providing Flynn with more than 22,000 pages of documents. The documents sought by Ms. Powell are irrelevant or had already been disclosed to Flynn ahead of his December 2017 guilty plea, he said.

Judge Sullivan asked both sides to file briefs on the materials Ms. Powell is seeking, setting a deadline of Oct. 15.

The Dec. 18 sentencing date comes on the first anniversary of Flynn’s previously scheduled sentencing. He abruptly requested to postpone sentencing after a very public rebuke by Judge Sullivan. At the time his lawyers said the delay was necessary so he could continue to cooperate with Mr. Mueller’s team

Flynn’s cooperation continued this summer when he was set to testify against his former business associate who was convicted of illegally lobbying on behalf of the Turkish government. But days before the trial was set to begin, prosecutors decided not to call Flynn, labeling him a co-conspirator.

Ms. Powell pushed back against the claim, accusing the government of being “disingenuous” in a court filing.

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