The 40-year-old actor's body was charred beyond recognition and lying "in a pugilistic stance" caused by the intense heat, they said. Walker was in the passenger seat of the car in the November 30 fatal accident.
"The driver was driving a red Porsche Carrera GT... at an unsafe speed, approximately 100+ mph," a full coroner's report said, citing an investigator's account of the crash.
The $400,000 car was driven by Walker's friend Roger Rodas, who also died in the crash in Santa Clarita, north of Los Angeles.
Describing Walker's body, the report said he was "wearing remnants of a black T-shirt, a pair of black jeans and a pair of grey boxer briefs" and was "lying supine in the passenger seat."
"The decedent was charred and in a pugilistic stance," it said, describing the posture of his body after the car span out of control and hit a tree and a light post before bursting into flames.
A coroner's spokesman said "pugilistic" described "how the muscles of the body contract due to the heat into a 'boxer-like' appearance."
It said Walker's body "cannot be positively identified visually," adding that "due to the extent of the injuries, the decedent (was) not (a) viable candidate for tissue donation."
The star's death stunned fans of the high-octane series "The Fast and the Furious", and was a major blow to studio giant Universal Pictures, for whom the franchise is a huge money-maker.
Universal subsequently announced the studio has shut down production of "Fast and Furious 7" following Walker's death.
The first "Fast and Furious" movie appeared in 2001. The series, with its focus on fast cars, tough guys, sexy starlets and exotic locales, is one of Hollywood's most successful global franchises.
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