Investigations into the crash which killed Fast and Furious star Paul Walker have concluded that the Porsche Carrera GT he was a passenger in had no mechanical problems and the car lost control due to speed, and speed alone.

Walker died from the combined effect of severe burns and injuries resulting from the crushing initial impact of the high-speed car crash, an autopsy revealed.

The driver of the Porsche was Roger Rodas, a former race car driver and financial adviser to Walker. Rodas had been close friends with Walker for several years and the pair had raced together on several occasions.

Both men were pronounced dead at the scene after their vehicle swerved off road, hit a tree and caught fire.

According to authorities, Walker was still alive when the car burst into flames, while Rodas was killed on impact. The victims were apparently so horrifically disfigured that the coroner had to use dental records to identify Walker and Rodas, and to determine which one of them was behind the wheel.

Walker, who began modelling as an infant in pampers commercials, shot to fame with the Fast & Furious franchise, starring in all but one of the six action blockbusters.

He had been working on the seventh instalment of the movie at the time of death.

Walker's younger brother, Cody Walker, 25, who closely resembles Paul, is expected to fill in for the late actor in the seventh edition of the movie, following deliberations by the film's production team. Cody Walker has previously worked as a movie stuntman.

"If Cody agrees it's because he wants to honour his brother's memory. There are many details that still need to be worked out, but right now the family and cast and crew are all still grieving," a source said.

"They can shoot Cody from behind and at distance. If it's a shot they need Paul's face in close up, they can CGI it later on," the source added.