“I know who it is,” Trump told reporters gathered at Andrews Air Force Base before departing on a trip to California.
Trump added that he can’t tell who it is, but he “knows who it is.”
“People know it’s a fraud,” he said. “I know about anonymous and I know about the leakers too.”
The anonymous author, who also penned a now-infamous op-ed in the New York Times in 2018 claiming that he or she was "part of the resistance" to undermine Trump, published a book entitled “A Warning” last November that was highly critical of the president. It slammed him for mocking immigrants and challenging major public policy decisions.
The author wrote in the book of an instance where Trump mocked the flood of migrants coming across the border.
"We get these women coming in with like seven children," Trump told his listeners, briefly attempting a Hispanic accent. "They are saying, 'Oh, please help! My husband left me!' They are useless. They don't do anything for our country. At least if they came in with a husband we could put him in the fields to pick corn or something," an excerpt from the book read.
The anonymous writer also enumerated several other instances where Trump took fiery and flippant stances on major public or policy issues and called the president's outward stance on combating corruption in Ukraine as "barely believable to anyone around him."
"The obvious corruption was in the Oval Office. The president had apparently learned nothing from the Mueller saga. Only we did. We learned that, given enough time and space, Donald J. Trump will seek to abuse any power he is given," the book reads.
The White House was quick to blast the book ahead of its release and attempted to undermine the author's credibility because of their decision to remain anonymous.
“The coward who wrote this book didn’t put their name on it because it is nothing but lies," White House press secretary Stephanie Grisham said in a statement on Nov. 7. "Real authors reach out to their subjects to get things fact-checked – but this person is in hiding, making that very basic part of being a real writer impossible. Reporters who choose to write about this farce should have the journalistic integrity to cover the book as what it is – a work of fiction."
The author's book includes a number of colorful anecdotes and details about their tenure working at the White House, painting a picture of Trump as "uncomfortable" to be around.
"He stumbles, slurs, gets confused, is easily irritated, and has trouble synthesizing information, not occasionally but with regularity," the book said.
The book also talks about Trump's alleged derogatory demeanor toward women, asserting that he "comments on makeup. He makes jokes about weight. He critiques clothing. He questions the toughness of women in and round his orbit. He uses words like 'sweetie' and 'honey' to address accomplished professionals."
The author also declares that less than halfway through Trump’s term “top advisers and cabinet-level officials contemplated what might be called a midnight self-massacre, resigning en masse to call attention to Trump’s misconduct and erratic leadership,” but the plan was scrapped, adding that a lot of staffers had "draft resignation letters in our desks or on our laptops."
“The idea was abandoned out of fear that it would make a bad situation worse. It got worse anyway,” the anonymous author wrote.
Fox News’ Vandana Rambaran contributed to this report.