SOUTH PORTLAND — A fire that displaced three families in a South Portland Housing Authority apartment building Sunday night started when someone improperly disposed of a cigarette, a fire department spokesman said Monday.

When the first firefighters arrived at 70 Grandview Ave. at about 10:10 p.m., the fire had already spread from one porch at the rear of the building to a second porch and into an electrical utility room, said South Portland Fire Lt. Robb Couture.

“They had a pretty heavy volume of fire when they arrived at the rear of the building,” Couture said.

The porches and utility room were heavily damaged by the fire, which had also climbed up the exterior of the two-story building. Smoke and water used to put out the fire further damaged three of the building’s four apartment units, he said.

The front of the building, visible from Grandview Road, had no signs of damage on Monday. At the rear, however, both porches showed signs of scorching, with some vinyl siding melted and more torn off the building and left on the ground among children’s toys and other wrecked possessions.

Firefighters had the fire under control within 20 minutes of their arrival, but they continued to rip away portions of the building overnight to make sure it was completely extinguished, Couture said.

Couture said only three of the building’s four units were occupied with the fourth vacant, and people were home when the fire broke out. He did not know how many people in total lived there.

“They did have working smoke detectors, so everyone got out safely,” he said.

Couture said he did not know the estimated cost of the fire damage or when the tenants could possibly move back in.

The South Portland Housing Authority was closed for Patriot’s Day. No one there answered the phone.