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Relatives of the Kill Bill star believe he was "assassinated" because he was trying to expose an underworld martial arts group.
The 72-year-old actor - who shot to fame in 1970s TV series Kung Fu - was found dead in a hotel in Bangkok, Thailand, last Thursday.
He was naked and had a rope tied around his neck, wrists, and genitals.
Thai police have denied murder claims, insisting the actor died after a bondage game went wrong, but his family's lawyer said there had been "conflicting reports" about the death.
Speaking on the Larry King Live US talk show, attorney Mark Geragos said: "David was very interested in investigating and disclosing secret societies - absolutely. What that means is connected to his interest in martial arts.
"And so there is a suspicion that if there was some foul play, that may be the first area they should look."
Carradine's manager Chick Binder also thinks the star was murdered, because his hands were tied behind his back, and there was a footprint on the bed that didn't match his shoes.
Binder said: "For sure there was foul play. We definitely don't believe it was a suicide. David was a great guy and a great client and a great friend."
But Marina Anderson - the fourth of Carradine's five wives - has dismissed murder claims, saying the actor often took part in "potentially deadly" kinky sex acts.
She said: "If he was involved in secret societies, it was a secret even I didn't know about."
Carradine's family has reportedly asked the FBI (Federal Bureau of Investigation) to send special agents from the US to Thailand to investigate the case.
They are also furious after a Bangkok newspaper printed a photo of the actor's body hanging from a rail in the wardrobe of the hotel room.
Carradine was fascinated with martial arts since he played wandering monk Kwai Chang Caine in 'Kung Fu'.
Nearly 30 years later, he played martial arts expert Bill in Quentin Tarantino's Kill Bill films.
If Carradine's death is connected to an underworld group, he would not be the first actor murdered in suspicious circumstances.
Conspiracy theorists believe martial arts expert Bruce Lee was murdered by Triads - an underground Chinese criminal organisation - in 1973.
- Bang! Showbiz.