An unoccupied classroom at Lopez Elementary School was left with a layer of soot after a malfunctioning heater thermostat apparently ignited a small fire in a heater motor late in the day Monday, according to school officials.

The fire, which video surveillance shows began at 5:48 p.m., burned for less than five minutes, but smoke left soot on the floor and walls on the unused classroom in an unused wing of Lopez, which this year became the district’s alternative school.

“The damage was discovered by a cleaning crew early this morning,” said Dr. David Spinks, the district’s director of student services. “Every parent has been notified about the incident. Students are not even in that part of the building during the day, and we’re in the process of cleaning the impacted area right now.”

Spinks said that although Lopez has video surveillance and audible smoke detectors, older schools in the district are equipped only with motion detectors to immediately alert authorities to after-hours incidents.