According to the Cal Fire San Bernardino Unit, the El Dorado Fire was caused by "a smoke generating pyrotechnic device" used during the party in El Dorado Ranch Park. The fire spread from the park to Yucaipa Ridge, which separates Mountain Home Village and Forest Falls from Yucaipa.
"Cal Fire reminds the public that with the dry conditions and critical fire weather, it doesn't take much to start a wildfire", the agency said in a press release. "Those responsible for starting fires due to negligence or illegal activity can be held financially and criminally responsible."
Some residents of the nearby community of Oak Glen, like Patrick Patterson, were upset to learn about the blaze's cause. His home has been surrounded by smoke since it ignited and says the flames got close overnight.
"That place, there's nothing green out there. It's a meadow of dry, brown, dead grass. Why would you go out there and think that you can light off any kind of firework? Smoke or otherwise. It all has fire," Patterson said.
CalFire investigator Capt. Bennet Milloy says the family that hosted the gender reveal party has been cooperating with authorities and they actually tried to put the fire out.
"They tried to use water bottles, which in four-foot high grass, you're never going to capture a grass fire with that," he said.
Still, they could potentially face serious consequences.
"Potential laws that they may have broken are some public resource codes, which is igniting a land that's not owned by you," Milloy added. "If you cause a fire on that land, that's a misdemeanor in the state of California."
There could also be misdemeanor or felony charges if the fire goes onto forest land, which it has, or if it destroys structures or other property.
"I am upset that people are stupid," said Forest Falls resident Linda Corcoran, who had to evacuate her home with her husband. "People don't possess common sense and so yes, I am angry, but not surprised."
The El Dorado Fire erupted Saturday before 10:30 a.m. in the 37000 block of Oak Glen Road. It has grown to 9,671 acres as of Monday evening and containment is listed at 7%.
Gov. Gavin Newsom declared a state of emergency on Sunday for San Bernardino County due to the fire. Four other counties in California were also under a state of emergency due to wildfires amid extreme temperatures.
WATCH: Time lapse video captures El Dorado Fire erupting near Yucaipa
Time lapse video captures El Dorado Fire erupting near Yucaipa
Evacuations were ordered when the fire erupted. According to the San Bernardino National Forest, the evacuation area included the communities of Oak Glen, Mountain Home Village, Forest Falls and North Bench Yuciapa (north of Carter St. to Highway 38 and both sides of Bryant St., east & west).
An evacuation center was set up at the Yucaipa Community Center at 34900 Oak Glen Rd. The evacuation center was later moved to Redlands East Valley High School at 31000 E. Colton Avenue.
Road closures were in place for Highway 38 at Bryant Street in Yucaipa and the community of Angelus Oaks, and Oak Glen Road at Pine Bench Road on the east and Cherry Croft/Jefferson on the west.
More than 600 personnel were battling flames, along with 60 engines, three fixed-wing aircraft and six helicopters, according to the San Bernardino National Forest.
No injuries have been reported. Officials did not immediately release details on structural damage.
California has seen 900 wildfires since Aug. 15, many of them started by an intense series of thousands of lightning strikes. The blazes have burned more than 1.5 million acres. There have been eight fire deaths and nearly 3,300 structures destroyed.
The heat wave was expected to spread triple-digit temperatures over much of California through Monday. Officials urged people to conserve electricity to ease the strain on the state's power grid.
The Associated Press contributed to this report.