U.S. intelligence officials reportedly overheard the conversation as part of their regular surveillance of Kislyak. When reports of the conversation became public in January, Vice President Mike Pence and then-White House Chief of Staff Reince Priebus publicly defended Flynn and denied any wrongdoing. Trump fired Flynn on February 14 for lying to both men about the nature of his communication with Kislyak.

Friday’s filing also said Flynn misled FBI investigators about his efforts to persuade the Russian ambassador on December 22, 2016 to delay a vote on a then-pending United Nations Security Council resolution about Israeli settlements.

Flynn’s statement confirms weeks of speculation that he would cooperate with the special-counsel investigation that has overshadowed the first year of Trump’s presidency. Flynn’s lawyers cut off communications with Trump’s lawyers earlier this month, a move that suggested he would potentially exchange information with Mueller’s team. In March, Flynn’s lawyer asked for immunity from prosecution in exchange for his client’s testimony, cryptically saying Flynn “certainly has a story to tell.” Trump, for his part, publicly encouraged him to do so at the time.

Mike Flynn should ask for immunity in that this is a witch hunt (excuse for big election loss), by media & Dems, of historic proportion! — Donald J. Trump (@realDonaldTrump) March 31, 2017

In the formal statement of the offense, filed Friday, the special counsel’s office laid out a timeline of Flynn’s conversations with Kislyak. After the Russian ambassador contacted him on December 28, 2016 to apparently discuss the recent sanctions, Flynn called “a senior official of the Presidential Transition Team” at Trump’s Mar-a-Lago resort in Florida for advice on a potential response.

“On that call, Flynn and the PTT official discussed the U.S. sanctions, including the potential impact of those sanctions on the incoming administration’s foreign policy goals,” the filing said. “The PTT official and Flynn also discussed that the members of the Presidential Transition Team at Mar-a-Lago did not want Russia to escalate the situation.” The identity of the official with whom Flynn spoke is not disclosed.

Flynn subsequently relayed those messages to the Russian ambassador and then notified the transition-team official of that conversation. Russian President Vladimir Putin announced his country would not retaliate against the Obama administration’s sanctions the following day. The filing also briefly describes a back-and-forth call between Flynn and Kislyak about the United Nations resolution vote one week prior at the behest of an unnamed “very senior member” of the transition team, as well as Flynn’s “other false statements” about his contacts and lobbying efforts on behalf of the Turkish government.