"The driver was driving a red Porsche Carrera GT ... at an unsafe speed, approximately 100+ mph,'' said the report.
"For unknown reasons, the driver lost control of his vehicle.'' The car struck a curb and several trees and burst into flames.
Walker's friend Roger Rodas, 38, was driving the car when it crashed. The limited-edition 2005 Porsche was built without a standard stability management system that most Porsche models are equipped with, according to the Los Angeles Times.
The car also has a history of being difficult to control, according to the newspaper.
The coroner's report, signed by Kristy McCracken, noted that Walker was burned beyond recognition, suffering "diffuse charring of the skin.'' The two men were identified by dental records.
Walker, 40, starred in the car-racing franchise Fast and Furious. Universal Studios has announced that it will put filming of the latest Fast and Furious movie on hold in the aftermath of Walker's death.
The franchise has grossed more than US$2.3 billion worldwide, with the last movie, Fast and Furious 6, the most popular with a gross of $789 million.