Cal Fire has opened an investigation into allegations that private firefighters set illegal backfires to protect properties threatened by the Glass Fire, a spokesman said Wednesday evening.

© Jessica Christian / The Chronicle

A hand crew monitors a backfire set behind homes to fight the Glass Fire burning along Tucker Road in Calistoga, Calif. Tuesday, September 29, 2020.

The agency’s law enforcement division is handling the investigation.

Cal Fire would not disclose where in Wine Country the private firefighters were apprehended, but confirmed that the incident occurred over the weekend as red flag conditions whipped up flames across Napa and Sonoma counties.

Private firefighters may remove flammable objects or carry out preventive measures, but are not authorized to use fire, said Cal Fire spokesman Scott McLean.

Many homeowners have taken extraordinary measures to protect properties from this year’s record-setting wildfires. Reports of people resisting evacuation orders to defend their homes and other structures have become increasingly common. Cal Fire officials repeatedly have warned that it is risky to both civilians and firefighters for people to ignore evacuation orders.

Private firefighters are used by insurance companies or homeowners to safeguard high-value properties from wildfires. They must receive permission from Cal Fire to enter a fire zone and are required to clear their activities with incident commanders, McLean said.

The exact location of the alleged illegal firefighting activity will not be made public until the investigation is complete, McLean said.

“We don’t generally have issues with them, but we stand our ground,” McLean said.