Three bodies were pulled from the wreckage of the plane. Two houses were destroyed in the crash, but nobody on the ground suffered injuries.
The crash comes less than a week after five people died in a plane crash at Melbourne's Essendon Airport.
"We still are actively searching for victims," local fire chief Michael Moore told reporters at the scene in Riverside.
"This involved two residences and reportedly they had residents in both homes."
There appeared to have been two adults and three teenagers on the plane.
The small plane had departed from Riverside heading to San Jose.
The cause of the crash is being investigated by authorities.
Riverside is located 90km southeast of Los Angeles.