Former national security advisor Michael Flynn pleaded guilty Friday to lying to FBI agents about his contacts with Russia’s ambassador to the U.S. – communications that a prosecutor said Flynn had discussed with “a very senior member” of President Trump’s transition team.

U.S. District Judge Rudolph Contreras said during the plea hearing at the federal courthouse that Flynn had agreed to provide “Substantial assistance for prosecution of another person.”

The guilty plea to a single count of making false statements to the FBI and the public reference to a “Very senior member” of the transition team marked the biggest step so far in the investigation headed by special counsel Robert S. Mueller III into Russian interference in the 2016 presidential election.

In a statement issued as he made his plea, Flynn, a retired Army lieutenant general, said he had wrongly been accused in public of “Treason and other outrageous acts.”

Trump’s lawyer, Ty Cobb, downplayed the plea and Flynn’s promise of cooperation.

Despite the courtroom statements about another member of the transition team, Cobb said in his statement that “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.”

Cobb also attempted to downplay Flynn’s role in the administration, saying that he was “At the White House for 25 days during the Trump Administration and a former Obama administration official.”

According to the plea deal, Flynn lied to FBI agents about whether he asked the Russian government in December 2016 to hold off on retaliating against sanctions imposed by then-President Obama for trying to interfere with the campaign.

Michael Flynn