WALLA WALLA, Wash. -

Police in Walla Walla, Washington, say two men have been seriously burned in an explosion that's believed to have resulted from a process used to extract hash oil from marijuana.

Officer Tim Bennett says 60-year-old Johny Ray Johnson and 32-year-old Thomas McCarthy have been airlifted to a Seattle hospital for treatment of second- and third-degree burns.

Two children and a woman at the rental property at the time of the Monday afternoon explosion were not hurt.

Bennett says the explosion collapsed part of the residence, and pushed exterior walls away from the foundation.

The extraction process involves the use of flammable butane.

The explosion is under investigation.