Former FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE on Tuesday said he was "surprised" to learn of his firing by President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE last year, questioning Trump's decision to intervene in the ongoing investigation into possible collusion between his campaign and Russia.

In an interview on "The Late Show with Stephen Colbert," Comey said that, despite a strained relationship with the president, he did not think Trump was planning to fire him.

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“I actually was quite surprised because I thought, ‘I’m leading the Russia investigation,’ ” Comey said, laughing. “Even though our relationship was becoming strained, there was no way I was going to get fired or whacked.”

"Why, why wouldn't you get fired?" host Stephen Colbert asked.

"Because that would be a crazy thing to do," Comey responded. "Why would you fire the FBI director who's leading the Russia investigation?"

"Because you're leading the Russia investigation!" Colbert shot back, laughing. "I don't know if you've dealt with mob bosses before, but they don't like being investigated."

Comey, who is on a media tour promoting his book "A Higher Loyalty: Truth, Lies, And Leadership," was fired last year by Trump in a decision that ultimately led to the appointment of Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE, the special counsel now overseeing the Russia investigation.

Trump on Wednesday responded to Comey with a morning tweet, dismissing the idea that Comey was fired over the "phony Russia investigation," despite the president's own previous comments stating he had weighed Comey's handling of the probe in the dismissal.

"Slippery James Comey, the worst FBI Director in history, was not fired because of the phony Russia investigation where, by the way, there was NO COLLUSION (except by the Dems)!" Trump wrote.

Comey's book, released Tuesday, is expected to be an immediate best-seller and had already topped Amazon's nonfiction best-seller list ahead of its release.