One apartment was destroyed and the tenants of a few more were displaced from a Town Center apartment building after a fire broke out in a top-floor unit Tuesday night.

The fire department responded to a fire alarm at the Ellsworth House, 2720 76th Ave. S.E., around 5 p.m. Tuesday. The probable cause of the fire was a heating pad that had been left on, according to Mercer Island Fire Department Battalion Chief, Les Kenworthy.

According to an apartment manager interviewed Wednesday morning, residents were evacuated for about two hours while firefighters put down the blaze.

“One unit is a total mess. The fire completely gutted it,” the manager said. “The heat was great enough to blow out the windows, but nobody was hurt. The residents responded responsibly.”

The residents of the burned unit were not in their apartment when the fire started but discovered it when they returned home. According to a report on the city’s Web site, the occupants discovered the room full of smoke, closed the front door and immediately advised other tenants to evacuate.

Both the building’s sprinkler system and automatic fire alarm worked properly, the report states. Firefighters contained the fire to the room of origin, and it was extinguished within 15 minutes. No injuries or fatalities occurred at this incident.

The Ellsworth House is a four-story structure that contains approximately 60 apartments, most of which are inhabited by older adults. The building is owned by Saint Andrew’s Housing Group, a local housing non-profit, and offers Section 8 housing. Ellsworth House was a high-priority project for Mercer Island because it is the only directly assisted lower-income housing in the city, according the ARCH, an organization created by Eastside cities and the county to preserve and increase low-to-moderate income housing.

Fire personnel fought the fire inside the structure. A total of 18 trucks were used with about 50 firefighters.

Since the structure houses many vulnerable adults and the sprinkler system does not cover the entire building, Island firefighters decided to activate what is termed a “strike team” response, said Kenworthy. A strike team response pulls in extra manpower and equipment from neighboring agencies, including extra aid cars and ambulances. Bellevue and Seattle firefighters also assisted. Two Metro busses were called in so that displaced residents could have a warm place to wait out the emergency.

Many residents found refuge in Roberto’s, an adjacent restaurant. Building occupants were not allowed back into the building for about two hours. As of Wednesday afternoon, nearly all residents had returned to the building with the exception of the residents who lived in damaged units. Mercer Island Youth and Family Services found housing for displaced residents.

The Ellsworth House is a wood frame structure with a masonry exterior. Preliminary damage estimates are $200,000.