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What happened on the day martial arts expert and Hollywood star Bruce Lee died in 1973 at age 32? Matthew Polly may have an idea.

Nearly 45 years after the film and fighting legend's sudden demise, the American journalist has published an intensive biography titled “Bruce Lee: A Life.”

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Polly interviewed over 100 people associated with Lee, including friends, family, colleagues, as well as his widow Linda Lee Cadwell and their daughter Shannon Lee.

While the official cause of Lee’s death was listed in his autopsy report as cerebral edema possibly caused by a sensitivity due to a painkiller called Equagesic, Polly suspects it was actually a heat stroke that may have taken down the kung fu idol.

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Steve McQueen pays respect to Bruce Lee's casket. (Bettmann/Getty Images)

“The key to understanding Bruce Lee’s death is that he collapsed 10 weeks before and almost died of the very same thing,” Polly explained to Fox News about his theory.

“On May 10, 1973 he walked into a small dubbing room on one of the hottest days of the month. They turned off the air conditioner to avoid ruining the soundtrack. He immediately overheated and got dizzy.

"He left the room and still collapsed to the ground. He got back up and when he walked into the heated room, he collapsed again and started violently convulsing. They got him to the hospital and the doctors suspected his brain was swelling… And so the first collapse looked exactly like a case of heat stroke.”

Cadwell herself told the Los Angeles Times in 1998 Lee’s death was caused by a brain swelling due to hypersensitivity caused by an ingredient found in Equagesic. However, Polly insisted Lee’s final moments during a fiery heat wave in Hong Kong mimicked more the symptoms he experienced before.

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Bruce Lee in "The Orphan." (Michael Ochs Archive/Getty Images)

“At the time there were dozens and dozens of rumors — that he was poisoned, that ninjas got to him, that someone put a death touch on him,” said Polly. “There’s no evidence of a cover-up. My conclusion is heat stroke… It’s a very common killer of young athletic men.”

Polly claimed that a couple months before the first incident, Lee had the sweat glands in his armpits surgically removed.

“He didn’t think they looked good on screen,” said Polly. “It was his job to look good on screen. [But] that would have made it harder for him to dissipate heat.”

But it wasn’t just the circumstances of Lee’s death that added to his mystique over the years. Lee died in the apartment of actress Betty Ting Pei, the last person who reportedly saw the star alive. It was Pei who reportedly offered Lee Equagesic after he complained of a growing headache.

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Bruce and Betty Ting Pei on the Hong Kong Coliseum studio set for "Way of the Dragon," June 1972. (David Tadman)

Polly, who interviewed the now 71-year-old, claimed Pei was not only Lee’s mistress but also allegedly the most significant affair of his life.

“That went on the longest and he started to be less discreet,” he said. “He would go out on dates with her in town where other people could see them… I think her whole life was marked by Bruce’s death. It’s a wound she never fully recovered from. I was struck by how intense her feelings for Bruce are.

"She basically said he was the love of her life. It was interesting to talk to this woman who was still in love with someone who died so long ago. Her feelings for him are still just as intense.”

Cadwell has not responded to Polly’s book. He did note her own memoir revealed that the now 73-year-old first considered it was a possibility Lee was unfaithful after he died in Pei’s bed.

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Steve McQueen and Sharon Farrell in "The Reivers," October 1968. (Bernd Lubowski/ullstein bild/Getty Images)

But his alleged romance with Pei may not have been the only affair Lee had. Polly claimed his relationship with Sharon Farrell, his co-star in 1969’s “Marlowe,” resulted in a rivalry with Steve McQueen.

“According to Sharon, who I interviewed, they hooked up and had quite a passionate romance,” siad Polly. “But Bruce was very dedicated to his wife and didn’t want to leave her. And so she ended it. In her next movie, ‘The Reivers,’ she co-starred with Steve McQueen… and she immediately hooked up with him.

"Bruce comes to the set and sees her. They have one last passionate tryst. She then tells him ‘We can’t continue this because I’m with Steve now.’ And Bruce becomes a little heartbroken. He tells her, ‘I understand, he’s a big star. But would you wait for me? I’ll be as big of a star too.’”

Farrell, who was enamored with Lee’s incredible physique, which he maintained with constant training and raw beef protein shakes, also revealed he “turned me inside out” and was also the love of her life.

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Left to right: Betty Ting Pei, Chuck Norris, Bob Wall and Bruce Lee at Golden Harvest studios during filming of "Way of the Dragon," June 1972. (David Tadman)

But there was one other relationship Lee happily enjoyed — and it was one reportedly introduced to him by McQueen himself.

“The one drug he really enjoyed was marijuana,” said Polly. “And it was actually Steve McQueen who turned him on to pot. Bruce quite enjoyed it.

"I was talking to some of his friends and they said he was always jumpy and bouncy. And it wasn’t until he had a couple of hash brownies that he would calm down. So I think in a way it was self-medication. It balanced him out.”

Before Lee met his own end after a brief, yet thriving Hollywood career where he happily indulged, he faced a tragedy of his own when his friend, celebrity hairstylist Jay Sebring, was brutally murdered in 1969 by the Manson family.

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Jay Sebring and Sharon Tate. (Getty)

“He was devastated,” said Polly. “As you know, Jay Sebring and Sharon Tate were two of the people killed in the Manson murders. And Sharon just lived a couple canyons away from where Bruce and his family lived.

"He was frightened and worried and apparently took extra precautions for a while… Some even thought he was paranoid. I think it’s something that affected him for the rest of his life… I think that’s because he saw what could happen with celebrities being targeted."

Film director and Tate’s widower Roman Polanski, a kung fu student of Lee’s, suspected the martial arts expert was the mastermind behind the horrifying murders that also took the life of their unborn child.

“At the crime scene, police found a pair of glasses and they thought it belonged to one of the killers,” said Polly. “So for the next couple of months, they couldn’t find any suspects. And Roman, who was grief-stricken, thought he could find the person who did it… He and Bruce were training after the murders. And one day, Bruce casually mentioned he lost his glasses and needed a new pair.

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Roman Polanski in recent years. (Reuters)

"For a moment, Roman’s heart seized. He said to Bruce, ‘I’ll tell you what. After the lesson, I’ll drive you to the store and buy you a pair as a gift. And the entire ride, Roman is thinking ‘Maybe it’s Bruce Lee.’ They get to the store and Bruce gives his prescription… Roman breathes a few sighs of relief because it wasn’t the same as what the police found… They remained good friends.”

Polly hopes his book will give readers fresh insight on the man behind the myth.

“When asked about his bad behavior in Hollywood, Bruce said, ‘I’m not saying I’m a saint, but I’m not as bad as the other guys,’” said Polly. “I think that was his view.”

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Bruce Lee (right) with James Franciscus. (ABC Photo Archives/Getty Images)