Fire spread through a Forest Lakes Elementary School classroom in the early hours of Thursday morning and officials believe a malfunctioning heating unit is to blame.

Around 1:20 a.m. Thursday, a Harford County Sheriff's Office deputy on patrol noticed a fire inside the school, located at 100 block of Osborne Parkway in Forest Hill, according to a release from the Maryland State Fire Marshal's Office.

Fire crews from Bel Air and Jarrettsville volunteer fire companies were called scene and quickly containted the blaze to the front area of a portable classroom and an outside heating unit where investigators believe the fire began, the release states.

While the portable classroom did have a fire alarm and sprinkler system, they did not activate, fire officials say.

The fire, smoke and soot caused about $40,000 damage to the building and $10,000 damage to what was inside it, according to the release.

The portable was being used strictly for storage purposes and was not an active classroom during the day, the release states.