Investigators have found no evidence that the Porsche carrying actor Paul Walker had mechanical problems before it crashed, killing the Fast & Furious star.
The investigation also ruled out debris or other road conditions as causing the car in which Walker, 40, was a passenger to slam into a lamppost and tree and burst into flames.
"We're looking at speed and speed alone," a law enforcement source said.
The November 30 crash also killed the driver, Walker's friend and financial adviser Roger Rodas, 38.
Mr Rodas and Walker co-owned a car racing team. Mr Rodas was also a professional driver who competed in 10 Pirelli World Challenge GTS races this year.
Investigators with the Los Angeles County Sheriff's Department have calculated a range of the speed at which they think the car was travelling, but will not disclose it until Porsche engineers come to California next month in the hope of extracting information from on-board data collectors.
The speed estimate is calculated through a formula that uses factors including the arc of tyre marks from the scene and how well the tyres would grip the road, said Chris Kauderer, chairman of the California Association of Accident Reconstruction Specialists.
The sheriff's office has said speed was a factor in the crash in an industrial park about 30 miles north west of central Los Angeles.
The car exploded in flames but its three data recorders survived and may produce information to pinpoint the speed. If they produce usable data, the recorders would be able to tell investigators whether seat belts were fastened, air bags deployed and whether Mr Rodas hit the brakes before impact.
Because the car is so rare, investigators will need Porsche's help to retrieve the data. A spokesman for the car company declined to comment.
Sheriff's investigators have also enlisted the help of California Highway Patrol accident reconstruction specialists, who tested the brakes on the wrecked car and found that at least one of the tyres was made in 2004.
The law enforcement source said the Porsche appeared to have negotiated a curve in the road successfully before crashing, saying: "They were well out of the curve when they lost control."