The El Dorado Fire above Yucaipa has scorched 7,386 acres and is 7% contained as of Monday morning.
Investigators say the fire was caused by a pyrotechnic, smoke-generating device at a gender reveal party in Yucaipa.
Fire authorities reminded the public that those responsible for starting fires can face financial and criminal consequences.
On Sunday, Governor Gavin Newsom issued a proclamation declaring a state of emergency for San Bernardino County due to the fire.
He issued similar declarations for San Diego, Fresno, Madera, and Mariposa counties for other fires as the statewide heat wave continues.
More than 600 firefighters and other personnel were at the scene Monday morning, aided by aerial tankers and helicopters.
The fire is above Oak Glen and the North Bench area of Yucaipa.