Four people — three adults and a child — were severely injured in Chino at about 8:30 p.m. Saturday when a large quantity of fireworks ignited and triggered an explosion.
Police said the blast on D Street engulfed one vehicle and set a second car ablaze; officers gave first aid before the victims were taken to local hospitals.
The Chino explosion was one of several July 4 weekend fire incidents across Southern California, including a Wilmington blaze that left one man in critical condition and forced 12 people from a hotel and home.
More than 70 firefighters also battled a Harbor Gateway commercial building fire late Saturday, while separate fires in Mission Hills and La Habra damaged vehicles and threatened nearby structures without reported injuries.
Investigators are still probing the causes, with officials in Wilmington already identifying commercial-grade fireworks near the burned vehicles.