The fire burning at a moderate rate near Smiley Boulevard and Vista Suelto Road was first reported around 3 p.m.
By 10:30 p.m., the fire was fully contained at 124 acres.
More than 200 firefighters from agencies including Cal Fire, Riverside County Fire Department and Redlands Fire Department were battling the blaze. Three tankers were attacking the fire from the air.
No injuries or damage were immediately reported.
Cal Fire investigators determined the cause to be illegal shooting. Anyone with information on the shooters was asked to call the tip line at (800) 633-2836.