The Divider: Trump in the White House, 2017-2021 by Peter Baker and Susan Glasser:
Flynn was making matters worse. On the same day in December that Obama imposed sanctions on Russia for its pro-Trump election interference, Flynn spoke by phone with Sergey Kislyak, the Russian ambassador, not once or twice but five times, at one point urging Moscow not to escalate in response to the sanctions and suggesting the new administration would reverse Obama’s tougher policy.
A week before the inauguration, the Washington Post columnist David Ignatius revealed Flynn’s contacts with Kislyak. Flynn insisted to Mike Pence, Reince Priebus, and Sean Spicer that he did not discuss sanctions with the ambassador, a reassurance that the vice president and press secretary then repeated publicly. But the Russian ambassador’s phone calls were routinely monitored, and FBI agents knew that Flynn’s denial was untrue. So just after the inauguration, they met with Flynn at the White House to ask about the conversations with Kislyak only to be surprised when he repeated that sanctions did not come up.