Fire officials in Southern California promise a full investigation of how and why a controlled burn became a dangerous wildfire.
Rich Hawkins, fire chief of Orange County's Cleveland National, says fire can sometimes stay alive underground in roots, even when the ground surface is cool to the touch and there's no smoke.
A 10-acre controlled burn set last Thursday was thought to be out.
However, the Sierra Fire, which erupted early Monday, broke out where the burn had been done four days earlier.
Hawkins says Santa Ana winds were not forecast when the controlled burn was set, but adds, "That's no excuse."
The 6,500-acre fire prompted the evacuation of more than 2,000 homes.
It's no longer considered a threat to homes and people have been allowed to return.
The wildfire is about 10 percent contained.