Notably, the filings do not mention other conduct by Mr. Flynn that has also attracted legal scrutiny. For example, he is also said to have left contacts with Russian officials off his application for a security clearance and again failed to disclose them in response to questions by officials processing that application, which could have resulted in separate false statement charges. Since the plea agreement only immunizes the conduct it mentions from further prosecution, it is not clear why such other issues were omitted.

Talking sanctions with the Russian ambassador

False Statements Regarding FLYNN’s Request to the Russian Ambassador that Russia Refrain from Escalating the Situation in Response to U.S. Sanctions against Russia 3. On or about January 24, 2017, FLYNN agreed to be interviewed by agents from the FBI (“January 24 voluntary interview”). During the interview, FLYNN falsely stated that he did not ask Russia’s Ambassador to the United States (“Russian Ambassador”) to refrain from escalating the situation in response to sanctions that the United States had imposed against Russia. FLYNN also falsely stated that he did not remember a follow-up conversation in which the Russian Ambassador stated that Russia had chosen to moderate its response to those sanctions as a result of FLYNN’s request.

Mr. Flynn has now admitted that he lied when he told the F.B.I. that he had not discussed sanctions with Sergey I. Kislyak, then the Russian ambassador to the United States, during the transition period. In fact, as Mr. Flynn has now acknowledged, after outgoing President Barack Obama signed an executive order on Dec. 28 imposing the sanctions on Russia in retaliation for its election interference, Mr. Flynn — at the request of an unnamed transition official — asked Mr. Kislyak for Russia to consider reacting with restraint.

Their conversations about sanctions have been well documented, and helped catalyze the chain of events that led to Mr. Mueller’s appointment. On Dec. 30, for example, after President Vladimir V. Putin of Russia announced that his country would not retaliate at that time for the sanctions, Mr. Trump wrote on Twitter: “Great move on delay (by V. Putin) — I always knew he was very smart!” On Jan. 15, Vice President-elect Mike Pence publicly denied that Mr. Flynn had discussed sanctions with Mr. Kislyak.

But soon after the F.B.I. interview in which Mr. Flynn falsely said the same thing, Sally Q. Yates, an Obama-era holdover serving as acting attorney general, warned the Trump White House that Russia could blackmail Mr. Flynn over having lied. Mr. Trump soon fired Ms. Yates over an unrelated matter, but did not act against Mr. Flynn until The Washington Post reported nearly two weeks later that intelligence officials knew that Mr. Flynn and the Russian ambassador had talked about sanctions, leaving Mr. Kislyak with the impression that the Trump administration would revisit the issue after taking office. Mr. Trump then fired Mr. Flynn, ostensibly for having misled Mr. Pence.

Mr. Trump later pressured James B. Comey, then the F.B.I. director, to go easy on Mr. Flynn, according to Mr. Comey, before eventually firing him over his handling of the Russia investigation — a step that led to the appointment of Mr. Mueller as special counsel.