Washington (CNN) Under fire from the White House, Michael Wolff defended raising questions in his new book about President Donald Trump's mental fitness as a responsibility of those who witness his behavior.

"It would be irresponsible not to," Wolff said Monday on CNN's "Tonight with Don Lemon."

The White House has slammed the book, "Fire and Fury: Inside the Trump White House," with press secretary Sarah Sanders calling it "complete fantasy," and an attorney for Trump sending a "cease and desist" letter to Wolff and the book's publisher.

CNN has not independently confirmed Wolff's assertions, which include claiming virtually everyone around Trump is concerned about the President's ability to do the job. Wolff t old CBS News earlier Monday he did not speak with Trump Cabinet members or Vice President Mike Pence.

Wolff said Monday that despite the pushback, "100%" of those around Trump are concerned about his mental fitness.

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