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 says there are aspects of the investigation into Russian election meddling that have not yet been revealed to the public, hinting in an interview that developments that occurred before his firing last year have yet to be reported.

In a Tuesday night appearance on "The Late Show with Stephen Colbert," Comey told the host that there are some things he couldn't include in his new book because it is classified.

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"Are there things that you know about the Russia investigation that were happening before you were fired that we haven’t learned yet as a public?" Colbert asked the former FBI chief.

"Yes," Comey responded.

"Can you tell me what those are?" Colbert continued, to laughs.

"No," Comey answered, chuckling. "No, and they're not in the book. I had to have my book reviewed by the FBI to make sure it didn't contain classified information or sensitive investigative information, so it's not in the book."

"OK," Colbert responded, motioning to a cup of red wine near Comey. "Drink some more wine."

Comey's book, “A Higher Loyalty: Truth, Lies, and Leadership," is expected to top both the Amazon and New York Times nonfiction best-seller lists, with the book's British-based publisher deciding to print an additional 700,000 copies due to high demand.

The former FBI chief was fired last year, with Trump saying at the time that Comey's handling of the Russia investigation was a factor. Comey's firing led to the appointment of special counsel 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, who today heads the Russia probe.

Trump has called Comey a “LEAKER & LIAR” over the revelations published in the book.

“James Comey is a proven LEAKER & LIAR. Virtually everyone in Washington thought he should be fired for the terrible job he did-until he was, in fact, fired,” Trump wrote on Twitter.