The book claims that Trump's youngest daughter and Donald Trump Jr.'s ex-wife were "inappropriately — and perhaps dangerously — close" to some Secret Service agents.
Leonnig wrote that Secret Service agents reported Vanessa had "started dating one of the agents who had been assigned to her family." Vanessa filed for divorce in March 2018, but it is unclear when the alleged relationship began.
Meanwhile, Tiffany "began spending an unusual amount of time alone" with a Secret Service agent after a breakup, according to the book.
Agency officials reportedly "became concerned at how close Tiffany appeared to be getting to the tall, dark and handsome agent," according to the outlets. The agent was reassigned, and it is unclear when the incident took place. Both Tiffany and the Secret Service agent said there was not an inappropriate relationship between them.
Leonnig wrote that, while it was unclear if Donald Trump was aware of what was said about his daughter and daughter-in-law by Secret Service members, he did want to remove staff he thought were too short or overweight for the job.
"I want these fat guys off my detail," he allegedly said, but may have confused office-based personnel with active agents, The Guardian noted. "How are they going to protect me and my family if they can’t run down the street?"
"The U.S. Secret Service is aware of an upcoming book which re-hashes past challenges the agency overcame and evolved from. Now and throughout its 156 year history, the agency's skilled workforce is dedicated to the successful execution of its critical protective and investigative missions," a Secret Service spokesman said.
A spokesperson for Tiffany told the Post that her "experience with the Secret Service was entirely professional" and that claims stating otherwise were "nothing more than gossip."
Meanwhile, a source told PEOPLE in March 2018 that Donald Jr. and Vanessa were living "separate lives," adding: "Even before all this Trump presidency stuff started, they weren't good and their marriage was strained."
According to Leonnig, the agent involved did not face disciplinary action as he and the agency were not official guardians of Vanessa at the time. Secret Service agents are prohibited from having relationships with those they protect out of concern that their judgement may be clouded by feelings.
Tiffany has since gotten engaged to Michael Boulos. The pair reportedly met at Lindsay Lohan's club in Mykonos in 2018, but began dating in 2019. She announced the engagement the day before her father left office in January.
Before his term ended, Trump reportedly extended Secret Service protection to his adult children for six months. While former presidents and first ladies are granted lifetime security, it is unusual for former first family members to receive protection if they are adults.
President Joe Biden would need to approve a further extension, but a Secret Service source told PEOPLE in January that the last-minute extension was granted by Trump as a "transition period."