Wildfires have burned more than 800,000ha in California this year, setting a state record even as crews battled dozens of growing blazes today in sweltering temperatures.
The unidentified couple chose the location to reveal their baby's gender.
They were accompanied by their young children and had a friend or relative videoing on a cellphone, said Captain Bennet Milloy of the California Department of Forestry and Fire Protection.
The family went into a field and fired off the device, which quickly ignited 1.2m grasses, Milloy said.
Conditions were perfect for a fire to spread quickly — triple-digit temperatures, low humidity, dry vegetation and a stiff breeze.
Surveillance video showed the couple frantically race to their vehicle to retrieve water bottles to try to extinguish the flames. It was futile and they called emergency services.
"You can't fight a fire like this with a water bottle," Milloy said. "They had no chance after it started."
Firefighters arrived within minutes and the distraught couple told them what happened and provided their own photos and video to aid the investigation, Milloy said.
"It's a pretty tragic situation," he said. "Obviously this was supposed to be a happy event."
The couple could be liable for the cost of fighting the fire and criminally charged.
In 2017, a massive Arizona wildfire was accidentally started by an off-duty Border Patrol agent who shot a target filled with an explosive blue powder. The fire caused US$8 million in damages, Agent Dennis Dickey was charged with a misdemeanour and sentenced to probation.
Last year, a homemade explosive used to reveal a baby's gender killed 56-year-old Pamela Kreimeyer in Knoxville, Iowa. The device was meant to spray powder but instead blew up like a pipe bomb. Kreimeyer, who was standing 13m away, died instantly when debris struck her head.
- AP