Authorities are now investigating at the Rosa fire in Riverside County and the much larger Santiago Canyon fire in Orange County and are offering a reward of $150,000 to find the arsonists.
At least two men have been arrested, and authorities questioned a man at the Santiago site Thursday, but would not tell CBS News why.
CBS News first learned yesterday that the massive Santiago Canyon Fire was being deemed arson by a task force of federal, state and local agencies. Officials said the fire had been set in two separate "points of origin."
The fire was set in a particularly strategic spot, said Orange County Fire Authority Chief Chip Prather.
"The persons or persons who did this were either exceptionally lucky or they have some knowledge about where you might want to do the most damage when you set a fire," he said.
"I wonder, frankly, if we didn't have this arson caused fire here, how many of our resources we might have had available to respond to other parts of the state."
The Santiago Fire has burned about 23,000 acres east of Irvine, officials said, and it is around 50 percent contained. Nine homes have been destroyed, with another eight homes and 12 outbuildings damaged. Four firefighters have suffered minor injuries fighting the blaze. As many as 3,000 people were evacuated.
In San Bernardino County, a man suspected of starting a small fire was arrested and another man was shot to death by police after he fled officers who approached to see if he might be trying to set a fire.
In Los Angeles, police arrested a man after witnesses say they saw him lighting a fire on a hillside.
Authorities say 41-year-old Catalino Pineda was seen starting a fire in the San Fernando Valley Wednesday and then walking away.
Witnesses alerted authorities and followed the man to a nearby restaurant where police arrested him.
Pineda was booked for investigation of arson. Authorities say the Guatemala native is currently on probation for making excessive false emergency reports to law enforcement.
Police and fire officials could not immediately say whether he might be connected to any of the wildfires in Southern California.