The 2005 Porsche Carrera GT carrying Walker and driven by Roger Rodas was traveling about 90 mph when the car slammed into a light pole in Valencia on November 30. The pole had a 45 mph speed limit sign attached.
The car began to drift coming out of a curve, causing loss of control, according to a report released Tuesday. The Los Angeles County Sheriff's Department and California Highway Patrol did the investigation with the help of Porsche.
The Associated Press reported in December that investigators had found no evidence that the car had mechanical problems and ruled out debris or other roadway conditions.
Investigators said the car's tires were nine years old.
Rodas and Walker had taken what was supposed to be a quick ride Nov. 30 from a fundraiser benefiting a charity that gives first-response aid to victims of natural disasters.
The Associated Press contributed to this report.