Steve Gillon said July 16, 1999 will forever haunt him.

That was the day when the plane of his pal, John F. Kennedy Jr., crashed into the waters off Martha’s Vineyard, killing him and his two passengers, wife Carolyn Bessette Kennedy and her sister Lauren Bessette.

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Gillon, who first met Kennedy at Brown University, wrote the upcoming biography “America’s Reluctant Prince,” which is scheduled to hit bookstores on July 9. It offers an in-depth reflection of Kennedy’s life based on personal conversations from over the years. It also highlights interviews with some of Kennedy’s closest friends, including some of those who are speaking out for the first time.

John F. Kennedy Jr. and his mother, Jackie Kennedy Onassis, in 1989. (Photo by Time Life Pictures/DMI/The LIFE Picture Collection via Getty Images/Getty Images)

John F. Kennedy Jr. and his mother, Jackie Kennedy Onassis, in 1989. (Photo by Time Life Pictures/DMI/The LIFE Picture Collection via Getty Images/Getty Images)

People magazine recently reported that in his book, Gillon described how on the afternoon of July 16, Kennedy, 38, checked the weather before he left the office for the airport in Essex, N.J, where he kept his plane.

The forecast indicated clear skies, which “was crucial since he was flying under visual flight rules rather than relying on instruments,” Gillon wrote. According to the outlet, Kennedy had only completed half of the lessons in his instrument training course at that time.

However, the weather deteriorated between the time Kennedy left the office and when he arrived at the airport. By the time his plane took off at 8:38 p.m., Kennedy was heading into a thick fog. People shared that shortly after 9:40 p.m., Kennedy became disoriented and his plane entered a downward spiral.

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John F. Kennedy Jr. and his wife Carolyn walk with their dog Jan. 1, 1997 in New York City. (Photo by Evan Agostini/Liaison)

John F. Kennedy Jr. and his wife Carolyn walk with their dog Jan. 1, 1997 in New York City. (Photo by Evan Agostini/Liaison)

“He should not have gone up that night,” Gillon told the outlet. “At the first sign of danger, he should have done what a lot of pilots did that night and flew inland, away from the ocean, spend the night somewhere and then pick up the next morning.”

Gillon said that this realization “was difficult to write.”

“It was [John’s] poor judgment that led to his death and the death of his wife and his sister-in-law, and there’s no way around that,” said Gillon. “John bears the responsibility of his recklessness that night and John alone. That is not easy for me to say, but when I wrote this book I decided my responsibilities as a historian superseded my responsibilities as a friend. The historical truth is what it is.”

But Gillon didn’t just focus on Kennedy’s tragic death. He also explored the many traumatic events that shaped the son of late president John F. Kennedy over the years. He also interviewed a psychologist at Columbia University who specialized in trauma to make sense of it all.

John F. Kennedy, Jr. gives his wife Carolyn a kiss during the annual White House Correspondents dinner May 1, 1999 in Washington, D.C. (Photo by Tyler Mallory/Liaison)

John F. Kennedy, Jr. gives his wife Carolyn a kiss during the annual White House Correspondents dinner May 1, 1999 in Washington, D.C. (Photo by Tyler Mallory/Liaison)

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“John experienced more death in his brief life than most people do,” said Gillon, noting his father’s assassination in 1963, as well as the killing of his uncle Robert Kennedy in 1968, among others.

“When you think about the trauma that John had, it’s not just his father’s assassination, it’s his uncle who became like a father figure, and then Aristotle Onassis, who was his stepfather,” explained Gillon.

“The trauma is compounded by the fact that John leaves the only house he ever knew, the White House,” he continued. “They moved to four different homes in the year after the assassination. And then there’s the Secret Service, a constant reminder of what happened to his father and that his own life might also be in danger.”

A young John F. Kennedy Jr. exploring his father's desk.

A young John F. Kennedy Jr. exploring his father's desk. (Getty)

“What I learned was that one of the ways people respond to that type of trauma is to seek out danger. Because they realize life can be snuffed out at any minute, they want to live life to the fullest. At the same time, they’re drawn to danger and the possibility of further trauma… [John] had escaped death and danger so many times… I think John just always believed something was going to save him, but it just didn’t that night. It’s just one possibility among many.”

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Gillon said that his book will shed new light on not just Kennedy’s death, but also the incredible life he briefly led.

“He will always be remembered for the promise that went unfulfilled,” said Gillon. “[John should] be remembered for who he was: a complicated and an extraordinarily decent human being.”

John F. Kennedy Jr. with his assistant Rosemarie Terenzio.

John F. Kennedy Jr. with his assistant Rosemarie Terenzio. (Courtesy of Rosemarie Terenzio)

Back in 2017, Kennedy’s former assistant Rosemarie Terenzio told Fox News she had a difficult time coping with her friend’s death. At the time, Carolyn was stressed by her husband’s non-stop schedule with George magazine, along with the growing attention from pestering paparazzi, compelling her not to attend a Kennedy family wedding. Terenzio said she convinced the 33-year-old to attend to avoid questions being raised on any reported marital woes. Carolyn would go on that fatal flight, which haunted Terenzio for years.

“It was very difficult,” Terenzio admitted at the time. “It took time for me to get back up and figure out what I wanted to do next… there was a lot of trial and error.”

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Terenzio chronicled her memories with Kennedy in the 2012 memoir “Fairy Tale Interrupted,” which helped her cope. Today she is the founder of a publicity firm. And while Terenzio’s career continues to flourish, she still thinks of her friends and what could have been.

“I don’t think he felt any pressure or felt any burden [to pursue politics] because of his name,” she said. “I think he saw it as a wealth of opportunity. And that he was very lucky. He thought about it. He certainly talked about running for office, but his priority was to make George a success.”