Although it has been two days since the deaths of Paul Walker and Roger Rodas, there doesn't seem to be a crystal clear answer as to what caused the unfortunate accident that claimed both of their lives.
As of now we know that the Porsche Carrera GT is a lot to handle, particularly with the stock tires it was equipped with — no indication of what tires this particular GT was wearing — and its obvious that speed was an issue in this matter. The debris field is too large for it not to have been.
The latest news concerning the Walker and Rodas crash is security camera footage from one of the areas commercial/industrial buildings. Though there is not a clear line of sight, you do see the tuft of smoke from the initial crash/break up of the automobile. From there the video footage speeds up and you see the large plume of smoke from the fire. It's clear by the end of the clip that plenty of bystanders made their way to the scene, either to gawk or in an attempt to help out first responders arriving at the accident.
1:14 The high speed crash that killed Paul Walker and Roger Rodas caused a huge explosion and fireball that could be seen for miles ... and it was all recorded on a security camera that captured the precise moment of impact.