(Reuters) - Former FBI director James Comey on Tuesday told late-night talk show host Stephen Colbert that the way U.S. President Donald Trump leads is “strikingly similar” to a mob boss.

A combination of file photos show U.S. President Donald Trump in the White House in Washington, DC, U.S. April 9, 2018 and former FBI Director James Comey on Capitol Hill in Washington, U.S., June 8, 2017. REUTERS/Carlos Barria, Jonathan Ernst/File Photos

Comey appeared on “The Late Show” on CBS to promote his tell-all memoir, “A Higher Loyalty: Truth, Lies and Leadership,” which went on sale at midnight on Tuesday.

Colbert asked Comey about how he described in the book the people around Trump as having a mob or a “Cosa Nostra” quality.

“The leadership style is actually strikingly similar,” he told Colbert.

“I don’t mean it in the sense that Donald Trump is out breaking legs or shaking down shop keepers. I mean it in the sense that (when) he leads, it’s all about the boss.”

Comey said that Trump appears to lack external reference points in his life, such as religion or history, necessary to be an ethical leader.

Comey added that Trump could be a more ethical leader if he surrounded himself with people who could serve as external reference points.

“But, I wouldn’t be optimistic,” said Comey, who appeared relaxed as he was dressed in a black shirt, slacks and a gray blazer.

Comey was fired by Trump in May last year as the Federal Bureau of Investigation was probing possible connections between Trump’s 2016 presidential campaign and Russia’s meddling in the U.S. election.

Referring to Comey’s sacking, Colbert asked him if he was surprised that he got “whacked.”

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

Russia has denied interfering in the election and Trump has denied any collusion or improper activity.

Conservative commentators, such as Fox News host Tucker Carlson, have attacked Comey as partisan and indecisive in his handling of the email scandal that dogged Hillary Clinton’s 2016 presidential campaign. Others, like former Harvard law professor Alan Dershowitz, have denounced Comey for leaking memos about his discussions with Trump.

For his part, Trump has repeatedly hurled insulting tweets at Comey, such as calling him a “slime ball,” in the run-up to the release of his book, challenging accusations made in the book and the author’s integrity.

“He’s Tweeted at me probably 50 times. I’ve been gone for a year,” Comey said. “I’m like a breakup he can’t get over.”