Neither man tested positive for alcohol or drugs, including opiates and methamphetamine, the coroner's report said, adding that autopsies were conducted on December 3, but toxicology results were only confirmed on Friday.
The report said the driver lost control of the car "for unknown reasons," causing it to spin around and hit a curb, after which the driver's side of the Porsche struck a tree and then a light pole.
"The force of those collisions caused the vehicle to spin 180 degrees and continue in an easterly direction. The passenger side of the vehicle then struck another tree and burst into flames," it said.
"The vehicle was totalled with major traffic collision damage all around, and it appeared that the vehicle was almost split in half," the report added, saying: "The majority of the vehicle was also charred."
It said Walker's body "cannot be positively identified visually".
Investigators found no mechanical problems with the Porsche or problems with the roadway.
Universal Pictures announced the studio had shut down production of Fast and Furious 7 following Walker's death.
- AAP