WASHINGTON ― President Donald Trump’s former national security adviser pleaded guilty Friday morning to lying to the FBI about his contacts with the Russian government, an extraordinary development in special counsel Robert Mueller’s investigation into Russian interference in the 2016 election.

Michael Flynn was charged with one count of making a false statement to FBI agents and appeared in federal court in D.C.

NEW: Flynn's statement about guilty plea. pic.twitter.com/8y4kgcTReA — Matthew Nussbaum (@MatthewNussbaum) December 1, 2017

In a criminal information filing from Mueller’s team, the government alleges that Flynn “willfully and knowingly made materially false, fictitious, and fraudulent statements” in an interview with FBI agents on Jan. 24. It alleges he falsely told the FBI that he did not ask the Russian ambassador to refrain from retaliating to sanctions the Obama administration imposed on Russia in late December. Flynn also allegedly lied about asking the Russian ambassador on Dec. 22 to delay or defeat a pending United Nations Security Council resolution.

Considering all of the additional charges Flynn could have faced in connection with his overseas dealings, the fact that he is facing only one charge is an indication that Flynn is offering significant cooperation and information that could further Mueller’s investigation.

Jonathan Ernst/Reuters Former national security adviser Michael Flynn arrives for a plea hearing at U.S. District Court in Washington, D.C.

The Associated Press and Washington Post reported Friday that Flynn admitted in his plea that he spoke to the Russians at the direction of Trump transition team officials.

In response to Friday morning’s charge, Trump lawyer John Dowd told Wall Street Journal reporter Rebecca Ballhaus: “I have no reaction. I don’t know enough. I’m not worried about it.”

White House lawyer Ty Cobb said the plea implicates only Flynn.

“The false statements involved mirror the false statements to White House officials which resulted in his resignation in February of this year,” Cobb said in a statement. “Nothing about the guilty plea or the charge implicates anyone other than Mr. Flynn. The conclusion of this phase of the Special Counsel’s work demonstrates again that the Special Counsel is moving with all deliberate speed and clears the way for a prompt and reasonable conclusion.”

The New York Times reported in late November that Flynn’s lawyer, Robert Kelner, was no longer sharing information with Trump’s legal team, further raising the possibility that Flynn was cooperating with Mueller’s probe. Kelner later met with the investigators, according to ABC News.

Flynn’s indictment follows Mueller’s team charging Trump’s former campaign chairman Paul Manafort and his former aide Rick Gates with 12 counts, including allegations of conspiracy against the U.S. and money laundering. Manafort and Gates pleaded not guilty to the charges and are scheduled to face a federal trial in May.

In October, the investigators also reached a plea agreement with former Trump campaign adviser George Papadopoulos, who lied to the FBI about being offered “dirt” on Hillary Clinton during the 2016 election. Papadopoulos also reportedly suggested a meeting between then-candidate Trump and Russian President Vladimir Putin last year.

But those indictments involved only Trump campaign officials. Flynn is the first former administration official to be charged, making it more difficult for the White House to distance the president from Mueller’s probe. After Trump fired then-FBI Director James Comey amid his investigation into the campaign, Comey testified that Trump had requested that he end the FBI’s investigation into Flynn. Flynn, who stepped down as Trump’s national security adviser in February after lying to administration officials about the extent of his communications with Russian officials, had long faced scrutiny from the multiple investigations into Trump’s campaign.

Jonathan Ernst/Reuters Flynn stepped down as Trump’s national security adviser in February after lying to administration officials about the extent of his communications with Russian officials.

Of particular interest was Flynn’s history of suspicious business dealings and financial ties to Russia, as well as his concealing that he worked as a foreign lobbyist for the Turkish government while advising Trump’s campaign.

In the fall of 2016, a businessman with close ties to Turkey’s autocratic President Recep Tayyip Erdogan paid Flynn more than $500,000 to conduct research aimed at discrediting the exiled cleric Fethullah Gülen. Flynn failed to disclose the work until March, after he had already stepped down as national security adviser.

Mueller’s team has reportedly investigated Flynn’s lobbying firm and its dealings with Turkey.

Flynn also hid multiple contacts with Russia during Trump’s transition, including discussing sanctions with then-Russian ambassador Sergey Kislyak. He also attended a meeting with Kislyak to discuss creating a backchannel with Putin. Also in attendance was Trump’s adviser and son-in-law Jared Kushner, who is now a senior White House adviser and also reportedly a subject of Mueller’s probe. White House officials have repeatedly denied any collusion between Trump’s team and Russia, often by downplaying suspected officials’ involvement in the campaign or the administration. In March, then-press secretary Sean Spicer diminished Flynn as just “a volunteer.”

Alana Horowitz Satlin contributed reporting.

This story has been updated with Flynn’s guilty plea and responses to his charges.