Two fires broke out in Southern California on Monday, critically wounding two firefighters and leading to 90,000 evacuations.
The larger blaze, the Silverado fire, had grown to more than 12,000 acres by noon on Tuesday local time.
A utility company said the fire may have been started by its equipment.
A spokesman for Southern California Edison said: "It appears that a lashing wire attached to a telecommunications line may have contacted SCE's power line above it, possibly starting the fire."
SCE is now investigating the incident, he added, and as a precautionary measure the company cut power to almost 19,000 homes and businesses.