The shocking recklessness that killed JFK Jr. and 2 of his friends

The teenager set off solo for a two-mile paddle across the bay. It was a chilly New England day, and the weather was worsening. A wave capsized his slender kayak, tumbling him into the frigid water.

He was hypothermic when he hauled himself up on the shore of Great Island. What could he do but break into a stranger’s home to take a hot bath? Safely warm and dry, he left a note to explain the burglary and signed it with his world-famous name.

“They got back to him, they were very nice about it: ‘Glad to be of assistance, John,’ ” writes William D. Cohan in “Four Friends” (Flatiron Books), out Tuesday.

After all, a broken window was a small price to pay for a visit from John F. Kennedy Jr.

“He couldn’t stop himself from being reckless,” says longtime friend Ed Hill, who like Cohan met Kennedy when all were students at Phillips Academy in Andover, Mass., better known as Andover — perhaps the most prestigious boarding school in America. “And he would have no trouble finding anyone to go along with any goddamn reckless amusement he could conceive.”

In that, the son of the murdered president had plenty of company at Andover. Cohan’s book, arriving in time for the 20th anniversary of Kennedy’s July 16, 1999, plane-crash death, is a lament for the lost — and a sobering reminder of the heedlessness so often born of privilege.

“The idea that we really were . . . part of some kind of young, invincible Delta Force, was intoxicating,” Cohan writes, as he traces the eerily similar paths of Andover classmates who died violently before they could make it past 40.

John — no one at school ever called him “John-John,” as most of America did — was “all wound up with nowhere to go” and full of nervous energy when he arrived at Andover in 1976, Hill remembers. He was an indifferent student, but he quickly gathered a tight coterie of party-loving friends.

“He was the most nonlinear thinker I knew,” says another chum, Gary Ginsberg.

By the time he started college at Brown University in 1979, Kennedy was “a fully formed, thrill-seeking adrenaline addict,” Cohan writes. Over the next few years, as he attended law school and tried multiple times to pass the bar exam, he came up with ever riskier adventures: a 125-mile kayak trip among the islands of the frigid Baltic Sea, a solo paddle through the Arctic, midnight rows in New York Harbor that barely skirted collisions with container ships.

One friend was convinced he had a death wish. “He just took so many crazy chances.”

Once Kennedy learned to fly, though, his flirtation with danger became a passionate affair.

“He literally wanted an escape from being on the ground,” Ginsberg says. “Going up into the clouds in the sky was a really important physical escape . . . he talked about the solitude of being up in the air.”

But the logical, linear mindset of a pilot “was so not the way John approached problem-solving,” Ginsberg says. “It was the last thing he should have been doing, given his intellectual bent.”

In 1996, Kennedy bought a “powered parachute,” a flimsy one-seat flying machine equipped with a huge fan that literally blew the contraption into the sky.

“I was afraid to fly that thing,” says lyricist and rancher John Perry Barlow, a mentor who took Kennedy on his first piloting adventures. “That was a very treacherous aircraft.”

Kennedy waved off the concerns. “The sunset is so beautiful up here,” he said after taking his first flight in it. “I have never done anything that compares to this.”

Three years later, he smashed it into a tree on Martha’s Vineyard, shredding his ankle so badly that he needed surgery — and a metal plate — to repair it.

Meanwhile, Kennedy earned his provisional pilot’s license and bought a plane, which he used to ferry himself and his new bride, Carolyn Bessette Kennedy, between New York and the family homes in New England — always, however, with a flight instructor at his side.

Six weeks after the parachute accident, as soon as the cast came off his ankle, Kennedy was back in the air — ignoring doctors’ orders — to fly his Piper Saratoga, with his wife and her sister aboard, to a family wedding in Hyannisport, Mass. This time, he left the instructor on the ground.

“He was really itching to get back up there,” Ginsberg says.

“It was magnificently stupid,” says Hill.

Kennedy was only authorized to fly using visual cues, not instruments. But he took off hours behind schedule, in dark and hazy conditions.

The plane went down in the waters off Martha’s Vineyard at 9:40 p.m. on July 16, 1999, killing all three instantly.

“All I could think was, You f–king idiot,” Hill says. “He died in a way that was sadly predictable. I was furious.”

The same sense of invincibility threaded through the lives of other Andover classmates. One was Will Daniel, the son of First Daughter Margaret Truman and New York Times managing editor Clifton Daniel, who never succeeded in making peace with his family legacy.

“I was always made to feel like being [President] Harry Truman’s grandson was the best part of me,” he once said.

“He drank to get out of pain, and he took a lot of chances,” says novelist Melissa Banks, his onetime girlfriend. “He put himself in situations, dangerous situations, as a result of that.”

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Daniel, a psychiatric social worker, specialized in tough projects — registering the homeless to vote, counseling teens with “explosive aggression.” But it was alcohol that led him to take a fatal risk.

After a night of drinking, Daniel — planning to use his mother’s Park Avenue apartment as a crash pad — crossed the thoroughfare against the light. A northbound taxi slammed into him, smashing his skull and putting him in a coma. His family took him off life support two days later.

Another alumnus, Harry Bull, was a dedicated sailor, an obsession he shared with his father and brother. They worked together in the family business, a major paper manufacturer in Chicago.

Bull derailed his college career with hard-partying in his first semester at Yale, but once he got it back on track, his daredevil days seemed to be behind him — except for one thing: his habit of cannonballing off the family sailboat for a swim without leaving anyone at the helm.

“He’d just dive over the side,” his brother Rick says — although it’s impossible to anchor a boat in the deep waters of Lake Michigan. “He didn’t see the danger.”

One Sunday in 1999 he took his daughters Maddie and Lexi, ages 7 and 5, out for a two-day sail. His wife stayed home with their infant son.

The three did not return.

Two days later, the Coast Guard found the sailboat, in perfect condition, drifting 27 miles offshore. It was fully stocked with life jackets and emergency gear.

Harry and Lexi’s bodies turned up soon after. Both had been strong swimmers. Both had drowned. Maddie’s remains were never found.

“I’ll bet Harry said to his girls, ‘Hey, let’s go swimming, let’s take a dip,’ ” Rick says.

Cohan means his book to be a meditation on “the fragility of life.”

“The greatest gift I could give my old friends was sharing with others the truths of their lives,” he writes. “Not even those who have every privilege that life offers, right from the start, can escape the inevitable.”

But it charts a darker theme: that Andover itself cultivated a deadly hubris in its elite charges.

“We really were ‘la crème de la crème de la jeunesse americaine,’ ” — the very best of America’s youth — “as we were told regularly,” Cohan writes. “It was no surprise, really, that people emerged from Andover thinking they could do … anything they wanted.”