An empty portable school classroom that someone had been living inside was damaged by fire Sunday.

The mobile classroom was one of a few stored by the Kennewick School District on the 500 block of West Railroad Avenue.

A passerby noticed some smoke coming from the portable and called 911 about 6:40 p.m. Sunday, said a release from Kennewick Fire Chief Chad Michael.

The area is in a strip of industrial buildings between Canal Drive and Columbia Drive in downtown Kennewick.

The first firefighters discovered heavy smoke coming from the empty portable that was sitting on concrete blocks, the release said. The portables are stored next to a large building owned by the school district.

Fire officials said it was clear someone had forced their way into the portable and was using it as a home. While firefighters believe someone caused the fire, the exact cause has not been determined.

The fire caused some interior damage to the older portable building, said Robyn Chastain, the district’s executive director of communication and public relations.