Story highlights Richard Clarke is a counter-terrorism expert who served US Presidents from 1992 to 2003

The only reason to hide activity from US intel agencies is because it's criminal, he said

(CNN) If President Donald Trump's son-in-law and his ex-national security adviser sought to set up a back channel with the Kremlin, the move would "(reek) of espionage and may even be illegal," a top counter-terrorism expert told CNN's Michael Smerconish on Saturday.

"If Michael Flynn were, in fact, there ... he certainly knew what he was doing," Richard Clarke said of claims the President's former national security adviser worked with son-in-law Jared Kushner and Russia's ambassador to the United States to establish a secret line of communication.

Flynn "had spent his career in the intelligence community; he ran an intelligence agency," said Clarke, who served as a security and counter-terrorism official under Presidents George H.W. Bush, Bill Clinton and George W. Bush. "There's no excuse for him to not know where espionage starts and diplomacy ends."

If the meeting happened, it also showed "monumental distrust" between the incoming administration and the intelligence community, Smerconish said.

"Well, it suggests they were trying to do something illegal," Clarke replied.

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