The investigation also has ruled out debris or other roadway conditions as causing the car in which Walker was a passenger to slam into a light pole and tree.
The car, a 2005 Porsche Carrera GT, is notoriously hard to handle.
"We're looking at speed and speed alone," a law enforcement official with knowledge of the investigation told The Associated Press. The official was not authorized to speak publicly about the ongoing investigation and spoke on condition of anonymity.
The Nov. 30 crash also killed the driver, Walker's friend and financial adviser Roger Rodas.
Rodas, 38, and Walker, 40, co-owned an auto racing team. Rodas also was 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 traveling, but they won't firm up that number until Porsche engineers come to California next month in the hope of extracting information from onboard data collectors.
The official would not disclose the current speed estimate. It is calculated through a formula that uses factors including the arc of tire marks from the scene and how well the tires would grip the roadway, said Chris Kauderer, chairman of the California Association of Accident Reconstruction Specialists.
The sheriff's office has said speed was a factor in the one-car crash in an industrial park about 30 miles northwest of downtown Los Angeles. A spokesman on Wednesday had nothing to add.
Though the car exploded in flames, 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 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.
The official said sheriff's investigators also have 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 tires was made in 2004. The CHP did not immediately respond to a request for comment.
The official told AP that the Porsche appeared to have negotiated a curve in the road successfully before crashing.
"They were well out of the curve when they lost control," the official said.