>> See the letter here
3. The warning: The Post reported Monday that former acting Attorney General Sally Yates, who was fired last month after she ordered Justice Department lawyers to stop defending the president's travel ban, told the Trump administration in January that Flynn "was potentially vulnerable to Russian blackmail" over his conversations with the ambassador.
>> Sally Yates reportedly warned White House that Flynn was ripe for Russian blackmail
4. The temporary replacement: Retired Lt. Gen. Joseph Keith Kellogg, who served in the U.S. Army from 1967 to 2003, reportedly has been named acting national security adviser.
5. What's next? Trump must decide on a permanent replacement for Flynn. According to The Associated Press, the top contenders are Kellogg; former CIA director David Petraeus, the retired four-star general who stepped down after sharing classified information with his biographer-turned-mistress; and retired Vice Admiral Robert Harward, a former deputy commander of U.S. Central Command and member of the National Security Council under President George W. Bush. The Times reported that Harward "is the leading candidate."
>>RELATED: Timeline of events, who will replace him?
– The Associated Press contributed to this report.