No one was injured in a fire at a Mountlake Terrace apartment Sunday night that displaced three residents and caused more than $110,000 damage.

The fire was reported at 6:20 p.m. by an alarm company receiving a system activation at the Lakeside Apartments in the 6000 block of 219th St. S.W. Firefighters responding for the fire alarm arrived to find heavy black smoke and flames inside a first-floor unit in a two-story building at the apartment complex. South County Fire crews had the fire under control within 10 minutes of arriving and kept the flames from spreading to other apartment units.

The residents were away when the fire started. Other residents of the building evacuated safely. No one was injured.

According to South County Fire spokesperson Leslie Hynes, fire investigators determined this was an accidental fire, most likely started by a mattress located too close to a baseboard heater. Many baseboard heaters are temperature-controlled and switch on when the temperature drops to the thermostat setting. To prevent fires, keep anything that can burn at least three feet from heating equipment.

Lakeside Apartments is one of 16 older multi-family buildings in Mountlake Terrace retrofitted with a new alarm system in 2017 as part of a code-compliance review conducted by fire officials following a fatal apartment fire in Everett. Like the building in Everett where the fatal fire occurred, Lakeside Apartments, built in 1969, had no alarm system prior to the 2017 upgrade.

“Without the early notification from the alarm system, it’s very likely this fire would have spread to more than one unit, putting more people and property at risk,” said Assistant Fire Marshal Karl Fitterer. “The alarm allowed residents to evacuate safely and firefighters to arrive in time to limit fire spread and damage.”