L-R: First Lady Melania Trump, President Donald Trump,former President Barack Obama, Michelle Obama at the US Capitol after inauguration ceremonies at the in Washington, DC, on January 20, 2017.

Former first lady Michelle Obama said she will "never forgive" President Donald Trump for spreading the so-called birther conspiracy theory with "reckless innuendos" that she said threatened her family's safety.

Michelle Obama wrote the scathing critique of Trump — among the harshest words she's ever had for the president — in her forthcoming memoir, "Becoming," according to a Friday report from The Washington Post, which received an advance copy. The book will be released Tuesday.

"The whole [birther] thing was crazy and mean-spirited, of course, its underlying bigotry and xenophobia hardly . But it was also dangerous, deliberately meant to stir up the wingnuts and kooks," Obama writes in the book, according to the Post. "What if someone with an unstable mind loaded a gun and drove to Washington? What if that person went looking for our girls? Donald Trump, with his loud and reckless innuendos, was putting my family's safety at risk. And for this I'd never forgive him."

When Trump was asked about Obama's book, he deflected and instead criticized his President Obama.

"I'll never forgive him for what he did to our United States military. I'll never forgive him for many other things," Trump told reporters outside the White House on Friday morning.

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The conspiracy theory, which alleged that former President Barack Obama was born outside the U.S. and therefore ineligible to serve as president, first surfaced in 2008. In 2011, Trump started pushing the theory in television interviews as he floated the possibility of a presidential run.

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"Why doesn't he show his birth certificate?" Trump asked in March 2011 on daytime talk show "The View."

On NBC's "Today Show" the same month, Trump said, "I would like to have him show his birth certificate, and can I be honest with you, I hope he can. Because if he can't, if he can't, if he wasn't born in this country, which is a real possibility ... then he has pulled one of the great cons in the history of politics."

Trump injected a religious element into the conspiracy, as well, suggesting that he hasn't shown his birth certificate because "maybe it says he's a Muslim."

The White House did not immediately respond to CNBC's request for comment.

The 426-page book is divided into three sections, the Post reports: Becoming Me, Becoming Us and Becoming More. The first discusses her upbringing in Chicago and explores issues of race, class and education. The second recounts her romance and relationship with Barack Obama, and the final section focuses on their lives in politics and under the public eye.

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