CONCRETE, Wash. - The Skagit County Sheriff's Office says Sunday's fire that destroyed an old school building in the town of Concrete appears to have been caused by three boys who were playing with lighters and caught an old mattress on fire.

Danielle Jensen

Fire destroyed the old Concrete school building.

One boy was 12 years old and the other two were 11. The boys were interviewed by deputies this week.

The sheriff's office says the boys thought they had extinguished the small blaze, but left with it still smoldering and the fire later reignited.

It has referred the case to county prosecutors for possible prosecution.

The fire gutted the three-story building, causing the roof and floors to collapse. No one was injured.

The building was built during the 1930s, but had been vacant for the past two decades.