Two youths were seen on top of Pleasant Green Elementary School a couple of hours before flames were spotted shooting out of the roof Saturday evening, says Todd Rasmussen, Granite School District police.

Salt Lake County firefighters arrived at the school, 8201 W. 2700 South, and battled the fire in what school personnel call the "pod" - a portable building with classrooms and a hallway connected to the main structure.Battalion Chief Dave Limberg said there was smoke throughout the building and approximately $15,000 in structural damage.

The fire had been burning awhile before flames were visible, Limberg said. The fire appeared to have burned from the roof down through a wall common to the hall and a classroom. Limberg said there was no logical reason for a fire at that location. "There was no ignition source."

The chief said kids could have stuffed rags into a vent on the roof and then set the rags on fire. He said he will have no definite answers until an investigation is completed.

Limberg said officials were having trouble getting into the building after the fire was out because of thick smoke still trapped in the building.

There was a tense moment when one firefighter attempted to hook a hose to the hydrant in front of the school. Limberg said the hydrant shot straight in the air in an apparent explosion; the firefighter was thrown onto his back by the force. The man was stunned but not injured, Limberg said.

Firefighters were able to use another hydrant across the street.

Pleasant Green's principal, Delbert Christensen, was out of town, but other officials said school will convene on Monday.