Almost exactly 20 years ago,John F. Kennedy Jr.‘s plane crashed into the Atlantic Ocean just off the coast of Martha’s Vineyard in Massachusetts — killing him, wifeCarolyn Bessette Kennedyand her sisterLauren Bessette.

An unbearable loss of three lives.

Gillon met John at Brown University in 1981 when he was a teaching assistant in a history class John attended. The two became friends, based on their mutual love of history and racquetball, which they often played together.

As he pieces together John’s final hours, Gillon describes in his book how on the afternoon of July 16, 1999, John checked the weather before he left the office for the airport in Essex, New Jersey, where he kept his plane.

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

But the weather deteriorated between when John left the office and when he arrived at the airport. After his plane took off at 8:38 p.m., he was heading into a thick fog.

Shortly after 9:40 p.m., John became disoriented and his plane entered a downward spiral.

“He should not have gone up that night,” Gillon tells PEOPLE. “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.”

Such a realization, Gillon says, “was difficult to write.”

From left: Carolyn Bessette Kennedy and John F. Kennedy Jr. in 1997.Dave Allocca/DMI/The LIFE Picture Collection/Getty

John F. Kennedy Jr. And Carolyn Bessette Kennedy

JFK JR

“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. John bears the responsibility of his recklessness that night and John alone,” Gillon says. “That is not easy for me to say, but when I wrote this book I decided my responsibilities as a historian superseded my responsibilities as a friend.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.”

As part of his research, Gillon explored the multiple traumas that John survived in his young life and interviewed a psychologist at Columbia University who specialized in trauma.

“John experienced more death in his brief life than most people do,” Gilllon notes: his father’s assassination as the nation’s 35th president but also the killing of his uncle Robert Kennedy, among others.

In speaking to the psychologist, Gillon wanted to understand how it may have affected John.

“What I learned was that one of the ways people respond to that type of trauma is to seek out danger,” he says. “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.”

RELATED VIDEO: JFK Jr. & Carolyn Bessette’s Marriage ‘Struggles Were Much More Real Than People Realize,’ Book Says

Members of the Kennedy and Bessette families embrace at the stern of the Coast Guard ship Sanibel as they prepare to travel to the U.S.S. Briscoe on July 22, 1999, for the burial at sea of John F. Kennedy Jr., Carolyn Bessette Kennedy and Lauren Bessette’s ashes.MATT CAMPBELL/AFP/Getty

Members of the Kennedy and Bessette families embra

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From left: John F. Kennedy Jr. and Carolyn Bessette Kennedy in 1999.Justin Ide/Getty

JFK JR. and Carolyn

John, according to GIllon, “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.”

Gillon stresses the purpose was not to psychoanalyze John but to better understand him.

“It’s just one possibility among many,” he says.

Now that so much time has passed, Gillons hopes his book will bring new understanding to John’s complex life.

“He will always be remembered for the promise that went unfulfilled,” says Gillon.

John, he says, should “be remembered for who he was: a complicated and an extraordinarily decent human being.”

source: people.com