U.S. President Donald Trump's former national security adviser, Michael Flynn, pleaded guilty Friday to lying to federal agents and has agreed to cooperate with investigators examining allegations of Russian meddling in the 2016 election and collusion between Moscow and the Trump campaign.
Appearing before a federal judge in a packed courtroom in Washington, Flynn, a 59-year-old retired army general, pleaded guilty to one count of making false statements to the Federal Bureau of Investigation about a series of private conversations he had in December 2016 with Russia’s then-ambassador to Washington, Sergey Kislyak.
The charge carries a sentence of up to five years in prison, but under U.S. sentencing guidelines the average sentence for the offense ranges from zero to six months.
The guidelines are advisory, but prosecutors agreed to seek a reduced sentence if Flynn provides “substantial assistance” with the investigation being led by special counsel Robert Mueller. No sentencing date was announced.
WATCH: Flynn arrives at court
As part of his guilty plea, Flynn agreed to "cooperate fully" with Mueller's team of investigators, answering questions, providing written statements, taking polygraph exams, and "participating in covert law enforcement activities." In return, Mueller's office agreed that Flynn "will not be further prosecuted criminally."
Flynn is the fourth member of Trump's 2016 presidential campaign to be charged by Mueller's team and the first former White House to plead guilty in connection with the Russia investigation.
WATCH: Former Trump Advisor Michael Flynn Pleads Guilty to Lying to FBI
On October 30, Paul Manafort, a former Trump campaign chairman, and Rick Gates, another senior campaign official, were charged in a 12-count indictment unrelated to the Russia investigation.
Another Trump campaign surrogate, George Papadopoulos, secretly pleaded guilty to lying to the FBI about his contacts with the Russian government and is cooperating with the special counsel.
Flynn's decision to cooperate with a probe that could implicate others close to Trump marks a dramatic turnaround for a man who staunchly campaigned for the real estate mogul and promised a hard edge in U.S. foreign policy before being fired for lying about his Kislyak interactions to Vice President Mike Pence.
Flynn didn't know at the time, but his phone conversations with Kislyak were all recorded by the FBI as part of its probe into Russian interference.
White House reaction
The White House sought to play down the significance of Flynn's guilty plea.
White House lawyer Ty Cobb said Flynn's plea does not implicate "anyone other than Mr. Flynn" and added Flynn was a "former Obama administration official" who served in the Trump White House for only 25 days.
But the plea agreement provided an indication that Mueller sees Flynn’s cooperation as critical to his investigation.
"The trick is we won't know, perhaps for some time, how significant it is," said Steve Vladeck, a professor of law at the University of Texas who closely follows the Russia investigation. "But it's a strong sign that more is coming. And what's coming down the pipe probably involves more senior officials and individuals closer to President Trump himself."
In a statement released after his court appearance Friday, Flynn said, "The actions I acknowledged in court today were wrong, and, through my faith in God, I am working to set things right."
Flynn was swept up in the Russia probe as the FBI began examining contacts between Russia and Trump campaign officials.
Flynn admitted to lying to the FBI about the conversations he had with the Russian ambassador at the behest of senior Trump transition officials shortly after the election.
The conversations focused on two foreign policy issues the Trump transition team sought to influence before coming into office: a pending U.N. Security Council resolution condemning Israel for its settlement activities in Palestinian territories and a possible Russian retaliation to sanctions imposed by then-President Barack Obama.
In two separate conversations -- December 22 and 23, 2016 -- Flynn, directed by a “very senior” member of the Trump transition team, called Kislyak to urge him that Russia “vote against or delay” the Security Council resolution. Kisliyak later called back to say Russia would not vote against the resolution.
'Very senior' member of transition team
Several U.S. news outlets have identified the "very senior" member of the transition team as Jared Kushner, Trump's son-in-law and senior adviser who is leading the White House's Israeli Palestinian peace efforts.
Five days later, on December 28, after Obama announced punitive sanctions against Russia over its interference in the election, Kislyak called Flynn, according to prosecutors.
The next day, Dec. 29, Flynn contacted an unnamed senior transition official who was at Trump’s Mar-a-Lago resort to discuss what to tell Kislyak about the sanctions.
The two discussed the impact of sanctions on Trump’s foreign policy. Immediately after the conversation, Flynn called Kislyak and urged him to "refrain from escalating the situation."
On Dec. 31, the day after Putin announced that Russia would not retaliate to the U.S. sanctions, Kislyak called Flynn to say that "Russia had chosen not to retaliate" in response to Flynn's request.
When confronted by the FBI four days after Trump's inauguration, Flynn, then the president's national security adviser, denied everything, according to court documents filed on Friday.
The filing also says Flynn falsely stated he did not remember Kislyak informing him the Kremlin had decided to "moderate its response to those sanctions" in response to Flynn's request.
The court document says Flynn also falsely claimed the Russian ambassador never described Moscow's response to that request.