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The Apple Fire burning in the San Bernardino Mountains north of Beaumont was sparked by a vehicle malfunction, authorities announced Monday.

Cal Fire investigators determined that a diesel-vehicle emitting burning carbon from the exhaust system malfunctioned, igniting the massive inferno Friday evening. That conclusion was drawn from multiple independent witnesses, as well as corroborating physical evidence, according to a statement from the Riverside County Fire Department.

The fire has scorched 26,850 acres – or roughly 42 square miles – as of about 9:30 p.m. Monday, according to an update from the InciWeb incident information website. The blaze grew by nearly 6,000 acres overnight and destroyed at least five structures by Monday night.

Containment on the fire, which began in Riverside County and has spread to parts of San Bernardino County, stands at 7%.

“Firefighters continue to work to slow the spread of the fire using all resources available including helicopters, fixed wing air tankers, ground equipment and hand-crews,” the U.S. Forest Service stated in the update.

A map showing the Apple Fire burn area was provided on the InciWeb incident information website.

The fire has prompted officials to issue evacuation orders to approximately 2,500 households, impacting roughly 7,800 residents.

Mandatory evacuations are in place for the Oak Glen community in San Bernardino County. Forest Falls, Pioneertown and Rim Rock are all under an evacuation warning. An evacuation center has been set up at Beaumont High School.

Riverside County residents can type their address on the county website to see if they’re affected by any orders. They can also sign up for alerts on

The fire is burning in a northeast direction, driven by dry brush, low humidities and high temperatures.

More than 2,500 firefighting personnel and nine helicopters are in the area battling the blaze.

“The Apple Fire is continuing to spread to the north across the head of the Mill Creek Canyon, and east into the San Gorgonio Wilderness,” Fire officials said in a 9 p.m. update. “As the fire burns into the wilderness the vegetation is becoming sparse limiting fire intensity. “

Officials did not provide information on the five structures destroyed and said damage assessment teams were deployed and will be surveying the fire area for damage to structures and infrastructure. Cal Fire previously reported at least one home and two outbuildings were destroyed near Avenida Miravilla.

One injury was reported Monday, but no information was available on the person hurt or the nature of the injury.

While investigators have determined the cause, Cal Fire officers are seeking information from anyone who may have seen a vehicle that appeared to have mechanical issues or unusual smoke on Oak Glen Road around the time the fire erupted. 

Anyone with information is asked to leave an anonymous tip by calling 800-633-2836.