Investigation underway into contacts between Flynn and Russian ambassador

WASHINGTON -- When President Trump left CIA headquarters on Saturday, a couple of steps behind him was his national security adviser -- retired General Michael Flynn. 

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Retired General Michael Flynn, who is President Donald Trump’s national security adviser.

CBS Evening News

CBS News has learned that U.S. investigators have been looking into at least one phone call -- in late December -- between Flynn and Russia’s ambassador to the United States, Sergey Kislyak.

The call came just as the Obama administration expelled dozens of Russian diplomats and announced sanctions in response to the wave of election cyberattacks.

On December 30, Russian Foreign Minister Sergei Lavrov suggested on Russian television that the Kremlin would retaliate by expelling American diplomats -- but shortly thereafter, President Vadimir Putin sent out a note saying that he would not kick U.S. diplomats out of the country, and instead talked about “restoring Russian-U.S. relations... based on the policies of the Trump administration.”

President-elect Trump then responded in a tweet: “Great move on delay (by V. Putin) -- I always knew he was very smart!”

Investigators are trying to determine whether the Trump team was coordinating with the Russians leading up to and after the election. So far, sources say, they have found no evidence of wrongdoing.

Today, White House Press Secretary Sean Spicer said the call between Flynn and the Russian ambassador covered four topics.

“One was the loss of life that occurred in the plane crash that took their military choir,” Spicer said. “Two was Christmas and holiday greetings. Three was to talk about a conference in Syria on ISIS. And four was to set up -- to talk about -- after the inauguration -- setting up a call between President Putin and President Trump.” 

Flynn is not the only Trump associate being scrutinized. Sources tell CBS News investigators are also trying to determine if others shared sensitive U.S. information with Russia, including corporate trade secrets, and if any criminal laws were broken.