Cal Fire spokesman Steve Kaufmann said the agency could not release further details about the cause, saying that could jeopardize the ongoing investigation.
In a statement released by Cal Fire, fire officials urged people living or visiting California to "be extra cautious with any activity that may cause a spark."
More than 1,000 firefighters today continued to battle the blaze that initially started off of Morgan Territory Road near Mount Diablo State Park, southeast of Clayton, on Sunday afternoon, according to Cal Fire.
The fire was at 90 percent containment as of this afternoon, fire officials said.
Fire crews expect to have the fire completely contained by Friday, and on Thursday a "significant demobilization" of fire crews was planned, he said.
The fire started off of Morgan Territory Road near Mount Diablo State Park, southeast of Clayton, on Sunday afternoon.
Since that time, more than 1,370 fire personnel from more than a dozen agencies responded to the blaze.
Today, some 1,160 firefighters were responding to the fire, Kaufmann said.
Crews are focused on holding the perimeter around the blaze and ensuring that the winds that tend to blow through the area in the afternoon don't cause any flare-ups, he said.
"Because the humidity is so low, we just want to make sure everything is out before we leave the area," Kaufmann said. "We are being extra cautious."
Kaufmann said the area engulfed in the wildfire, dubbed the Morgan Fire, is covered with vegetation left dry from an ongoing lack of rain and low humidity.
Since grasses, trees and other fuel in the area were already dried out, the fire was able to move much more quickly than it would during a year with more rainfall, he said.
Both in the Morgan Fire and in wildfires throughout the state, he said, "We're seeing fire behavior that's super extraordinary, because it's so dry and those fuel beds are so receptive to fire."
Residents from as many as 100 area homes threatened by the fire have been allowed to return home since evacuations were ordered Sunday.
Kaufmann said the Morgan Fire serves as a reminder that residents should be prepared to evacuate immediately in the event of a wildfire before crews arrive in their neighborhoods. Equally important, he said, is for residents to secure their homes, closing all windows, doors and garages to prevent fire from spreading to the building.
"When fire is approaching neighborhoods, it's real important for residents to make sure their houses are locked up tight," he said.