A piece of equipment caused the Steele Fire in July that burned across 135 acres, destroyed eight homes and damaged another eight, according to fire investigators.
It's unclear what kind of machine started the fire, said Cal Fire/Napa County Fire Capt. Justin Hartman. A full report on the fire may be released in the coming weeks, he said, but it's not known whether that report will contain more specific details about the cause.
Cal Fire responds each year to 1,600 fires started by people incorrectly using equipment. Hartman pointed to safe equipment use guidelines posted on Cal Fire's site.
The public is encouraged to avoid using yard maintenance equipment with an electric or gas motor past 10 a.m., when the day is hotter and the wind blows stronger.
Lawn mowers, tractors, chainsaws and motorcycles are examples of the kind of machines that could cause an equipment fire, according to the Cal Fire guidelines.
People who are found to have caused a fire could face criminal or civil penalties, according to Shana Jones, chief of the Cal Fire unit that encompasses Sonoma, Lake and Napa counties.