Retired Army Lt. Gen. Michael Flynn, center, arrives for sentencing at federal court in Washington, with attorney Robert Kelner, right, on Dec. 18, 2018. Flynn pleaded guilty to lying to the FBI during the course of special counsel Robert Mueller's investigation into possible collusion between the Trump campaign and the Russian government. (Photo: Diego M. Radzinschi/ALM)

Former Trump administration national security adviser Michael Flynn has split with his Covington & Burling legal team as he awaits sentencing on charges that he lied to the FBI.

In a court filing Thursday, Covington attorneys said Flynn was “terminating” his defense lawyers at the firm and had “already retained new counsel.” The revelation came in a motion, filed by Covington partners Robert Kelner and Stephen Anthony, to withdraw from Flynn's case in Washington federal court.

A Covington spokesman declined to comment. It was not immediately clear whom Flynn had hired to take up his defense, or why Flynn wants to split with his legal team.

Flynn pleaded guilty in 2017 to lying to the FBI about his discussions during the presidential transition with Sergey Kislyak, at the time Russia's top diplomat in the United States. He was set to be sentenced in December, with prosecutors backing his request for leniency.

But, in a dramatic two-hour hearing, he asked to delay his sentencing after U.S. District Judge Emmet Sullivan of the District of Columbia signaled that he could not rule out prison, and questioned whether Flynn could have faced more serious charges.

At the time, Kelner said the delay would allow Flynn to “eke out the last modicum of cooperation” in his deal to assist in a separate case being prosecuted in Alexandria, Virginia, involving two of Flynn's former business associates.

The notice of Covington's firing came during a period of heightened interest in Flynn's case.

Last week, prosecutors rebuffed an order to make public the transcripts of Flynn's conversations with Kislyak, in which the two discussed sanctions the Obama administration imposed on Russia for its interference in the 2016 election. Prosecutors, resisting Sullivan's order, said the government “represents that it is not relying on any other recordings, of any person, for purposes of establishing the defendant's guilt or determining his sentence, nor are there any other recordings that are part of the sentencing record.”

Sullivan told prosecutors on Tuesday that they did not have to make public the transcripts of Flynn's calls with Kislyak. At the end-of-May deadline, prosecutors complied with a separate order to publicly release the transcript of a voice mail Kelner received in November 2017 from John Dowd, then a personal lawyer for President Donald Trump. The call from Dowd came after Flynn withdrew from a joint defense agreement with Trump, signaling that he'd begun cooperating with Mueller's office.

Listen to the voice mail:

In the transcript, much of which was provided in the 448-page report summarizing Mueller's investigation, Trump's lawyer is quoted discussing the president's “feelings toward Flynn.” The voice mail was examined as part of Mueller's investigation into whether Trump or his associates sought to obstruct the Russia investigation.

Mueller's office found no evidence of coordination between the Trump campaign and Kremlin but did not clear the president on the obstruction charge. “If we had had confidence that the president clearly did not commit a crime, we would have said so,” Mueller said last week in his first public remarks on the two-year investigation.

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