Former Trump national security adviser Michael Flynn on Friday pleaded guilty in federal court in Washington to lying to the FBI in January about his contacts with the Russian ambassador, and said he is cooperating with Special Counsel Robert Mueller.

Flynn, 58, is charged with one count of “willfully and knowingly” making “false, fictitious and fraudulent statements” to the FBI regarding conversations he had with then-Russian Ambassador Sergei Kislyak days after President Trump was inaugurated.

In a statement, Flynn rejected accusations of treason, but that he acknowledged that what he did was "wrong."

“But I recognize that the actions I acknowledge in court today were wrong […] I am working to set things right,” he said.

Flynn added: “My guilty plea and agreement to cooperate with the Special Counsel’s Office reflect a decision I made in the bests interests of my family and of our country. I accept full responsibility for my actions.”

Flynn becomes the first Trump administration official to be charged as part of Mueller’s far-ranging probe into Russian interference in the 2016 presidential election and possible collusion with the Trump campaign.

Four Trump-connected associates in total have been charged as part of Mueller’s investigation. The other three were involved with Trump's campaign, not his administration.

In October, Mueller's team charged Trump campaign foreign policy adviser George Papadopoulos’ with making false statements to the FBI regarding his contacts with Russian government officials during his time on the Trump campaign. He pleaded guilty.

Then last month, former campaign chairman Paul Manafort and his business partner Rick Gates were indicted on charges ranging from money laundering to making false statements, and have since pleaded not guilty.

Flynn, a retired Army lieutenant general, was fired by the White House in February after it was revealed he misled Vice President Mike Pence about his communications Kislyak. The Department of Justice had warned the White House that Flynn could be blackmailed because he had not fully disclosed multiple contacts and payments from foreign entities, including Russia, before joining the administration.

The charging document says Flynn lied to the FBI on Jan. 24 when he denied asking Kislyak on Dec. 29 "to refrain from escalating the situation in response to sanctions that the United States had imposed against Russia that same day."

The document also says Flynn lied about a Dec. 22 conversation when he said he didn't ask Kislyak "to delay the vote on or defeat a pending United Nations Security Council resolution."

Following Flynn’s firing, it was reported President Trump asked former FBI Director James Comey to think about “letting Flynn go.” It was believed that Comey’s comments about this conversation led Trump to fire Comey in May.

Following Comey’s firing, Attorney General Jeff Sessions took over the Russia investigation, and later recused himself after disclosing his own Russian contacts during his time as a Trump adviser.

Deputy Attorney General Rod Rosenstein appointed Mueller as special counsel on May 11.