A firework at a gender reveal party triggered a wildfire in southern California that has destroyed 7,000 acres (2,800 hectares) and forced many residents to flee their homes, the fire department said Sunday.
More than 500 firefighters and four helicopters were battling the El Dorado blaze east of San Bernardino, which started Saturday morning, California Department of Forestry and Fire Protection (Cal Fire) said.
Residents of several communities in the area have been ordered to evacuate.
"CAL FIRE Law Enforcement has determined the El Dorado Fire, burning near Oak Glen in San Bernardino County, was caused by a smoke generating pyrotechnic device, used during a gender reveal party," Cal Fire said on Twitter.
"Those responsible for starting fires due to negligence or illegal activity can be held financially and criminally responsible," it warned.
Gender reveal parties are held during pregnancy to unveil the sex of the expected child, sometimes announced by pink or blue smoke fireworks.
With hot and dry conditions, California has suffered a particularly busy fire season this year, with more than 1.6 million acres destroyed so far, including in three of the biggest blazes in the state's history.
In northern California, more than 200 people had to be airlifted to safety over the weekend after a fast-moving wildfire trapped them near the Mammoth Pool Reservoir northeast of Fresno.
The Creek Fire, which started on Friday in steep and rugged terrain, has so far spread to 45,500 acres, according to the US Forest Service, making it one of the largest blazes.
A rapidly-spreading fire near San Diego, Valley Fire, has so far spread to 10,000 acres and destroyed 11 structures, Cal Fire said.
Record temperatures over the three-day Labor Day weekend have aggravated already dangerous fire conditions and further stressed exhausted California firefighters.
Temperatures reaching more than 110 degrees Fahrenheit (43 Celsius) were forecast for the Los Angeles area on Monday.