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 said during an interview on Sunday he has serious doubts about 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's credibility whether he is under oath or not.

"I have serious doubts about his credibility," Comey told NBC's Chuck Todd on "Meet the Press."

"Whether he were under oath or not?" Todd asked.

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"Correct," Comey responded. "And sometimes people who have serious credibility problems can tell the truth when they realize that the consequences of not telling the truth in an interview or in the grand jury would be dire. But you'd have to go in with a healthy sense that he might lie to you."

Comey, who was fired by Trump last year, is in the process of promoting his new book, "A Higher Loyalty: Truth, Lies, and Leadership."

The president fired Comey last May, purportedly due to his handling of the probe into Hillary Clinton's use of a private email server during her tenure as secretary of State. But Trump later acknowledged that the FBI's investigation into Russian meddling in the 2016 presidential election also factored into his decision.

Trump has repeatedly called Comey a liar and a leaker for authorizing the disclosure of personal memos detailing his interactions with the president.