Two school buses were destroyed by fire early Wednesday morning at Chippewa Valley High School, on the grounds of the Northern Wisconsin Center for the Developmentally Disabled in Chippewa Falls.

The Chippewa Falls Fire Department was called to the fire at 5:08 a.m. Someone at the Chippewa Valley Correctional facility, across the street from the center grounds, smelled smoke and drove around to find the source.

When firefighters arrived, the first bus was fully engulfed in flames, and a second bus parked next to the first was also on fire.

“We think the cause was they were both plugged into block heaters,” Fire Chief Tom Larson said.

The 2005 Chevrolet mini-buses are owned by CESA 11 out of Turtle Lake. They were used to bus children to a Head Start program at the alternative school on the Northern Center campus, Larson said.

The buses had probably been parked at that location for days as the Chippewa Falls School District is in the middle of a 12-day holiday vacation, and school has not been in session. A third bus was that parked along side the first two was untouched by the fire.

Larson said both buses were plugged into block heaters to keep the engines warm.

“For some reason, it must’ve shorted out,” he said.

The heaters are commonly used to keep engines warm throughout cold winters and are generally safe, Larson said. But he added that with all electrical equipment, there is risk of fire.

The damage estimate was unknown early Wednesday morning.

“Two of them are destroyed, and it’s probably a very expensive fire,” Larson said.

Magnesium was released from either the engine or mechanical components of the bus, making the fire hard to fight at times. The magnesium caused flare-ups, and Larson said it was evident that the fire was burning hot as metal was melted.

A foam mixture was used to quash the flames.