Michael T. Flynn, the national security adviser, resigned on Monday night after it was revealed that he had misled White House officials about conversations he had with the Russian ambassador to the United States.
President-elect Donald J. Trump offered Mr. Flynn the post of national security adviser, elevating a retired general and intelligence officer who sees Islamist militancy as a global existential threat.
“I am pleased that Lt. Gen. Michael Flynn will be by my side as we work to defeat radical Islamic terrorism, navigate geopolitical challenges and keep Americans safe at home and abroad,” Mr. Trump said in a statement at the time.
In a phone conversation with Sergey I. Kislyak, the Russian ambassador to the United States, Mr. Flynn discussed sanctions that the Obama administration had imposed on Russia around the same time, according to officials who saw a transcript of the wiretapped conversation.
Officials said Mr. Flynn never explicitly promised that the sanctions would be relieved but seemed to leave the impression that it was possible. The sanctions were imposed after intelligence agencies determined that President Vladimir V. Putin of Russia had tried to interfere with the 2016 election to help Mr. Trump.
After the call, Mr. Flynn told Vice President-elect Mike Pence and others that it was limited to small talk and pleasantries, leading Mr. Pence to use false information to defend Mr. Flynn in television interviews. As a result, officials said, Mr. Pence felt he had been lied to.