CHICAGO (CBS) — Five school buses caught fire Friday morning in north suburban Lake Villa.

Three of the buses were completely engulfed in flames and destroyed, and two others were damaged. An employee at Durham School Services saw the buses on fire around 5 a.m. at a bus yard in the 200 block of West Park Avenue.

Lake Villa Fire Protection District fire inspector Greg Phillips said the fire appeared to be an accident.

“It was just determined to be an electrical issue with the block heater,” he said.

Phillips said it’s routine for diesel buses to have a block heater plugged in at night in winter to keep the oil in the engine warm, and make it easier to start the buses in the morning.