Story highlights Father: The boys were apparently trying to start a fire in the fireplace

They may have used fuel from a school science project

The boys suffered fragmentation wounds

It started with a school science project. It ended with a house explosion so powerful it busted windows and blew open the back door.

Two boys were hospitalized Sunday after the blast at a Seattle home, police said.

One of the boys' fathers said his son had been working on a school science project involving model rocketry. He said the boys apparently were trying to start a fire in the fireplace and may have used some of the fuel intended for the rocket, Seattle police Detective Renee Witt said.

The explosion broke out windows in the living room and kitchen and propelled some debris out into the back yard, Witt said.

The boys, who were cousins, suffered wounds.

Investigators sifted through the living room and found several homemade fireworks bottle rockets, small aerial display mortars and a skyrocket.

"There was a supply of items to build many more, including assorted chemical powders for mixing up pyrotechnic powders," Witt said. "The completed items were seized for destruction."