Former White House staffer Omarosa Manigault-Newman said President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE's mental decline "could not be denied" while he appeared for an interview with NBC News's Lester Holt in 2017, according to an excerpt in her new book obtained by the Daily Mail.

"While watching the interview I realized that something real and serious was going on in Donald's brain," she writes in "Unhinged: An Insider's Account of the Trump White House," which is set to be released on Aug. 14.

"His mental decline could not be denied."

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Manigault-Newman added that "many didn't notice it as keenly as I did because I knew him way back when. They thought Trump was being Trump, off the cuff. But I knew something wasn't right."

Her comments were published by the Daily Mail over a year after Trump sat down with Holt for an interview that quickly grabbed Washington's attention. Among other things, Trump said that “regardless of recommendation" he was going to fire former FBI Director James Comey James Brien ComeySteele Dossier sub-source was subject of FBI counterintelligence probe Judge will not dismiss McCabe's case against DOJ Democrats fear Russia interference could spoil bid to retake Senate MORE.

The White House had previously claimed that a Justice Department recommendation had led to the FBI chief's ouster.

In her book, Manigault-Newman writes that "throughout this erratic and contradictory interview, I kept thinking, 'Oh no! Oh no! This is bad!'"

"Donald rambled. He spoke gibberish. He contradicted himself from one sentence to the next.

"Hope [Hicks, then communications director] had gone over the briefing with him a dozen times hitting the key point that he had fired Comey based on the recommendation by the DOJ which the vice president and other surrogates had been reinforcing for days."

Manigault-Newman, who was previously a contestant on Trump’s reality show “The Apprentice," left the Trump administration last year. The White House later said she was fired, but Manigault-Newman denied the account.

The former Trump staffer touted her forthcoming book in a tweet last week with the hashtag, "ComeClean45."

The newly revealed comments from Manigault-Newman take on a different tone than what she exhibited while working with Trump. During the presidential campaign, she said that “every critic, every detractor, will have to bow down to President Trump.”

Gallery Books, Manigault-Newman's publisher and a division of Simon & Schuster, told the Daily Mail that Manigault-Newman and Trump's relationship "has come to a decisive and definitive end, and Omarosa is finally ready to share her side of the story in this explosive, jaw-dropping account."