From staff reports:

South King Fire and Rescue responded to an explosion at 5:30 p.m. Friday in a house at 30431 23rd Ave. SW in Federal Way. Four adults were injured in the explosion. Two occupants were transported to Harborview Medical Center with serious injuries, and two occupants were treated and released at the scene.

The fire district reports that the home had a natural gas leak, which likely caused the explosion. A source of ignition was found in the basement. Emergency crews found extensive damage from the explosion and initial fire ball. In searching for any remaining fire, firefighters found a fire in an attic wall, according to a report.

An investigation is under way by the fire district and Puget Sound Energy. Damage has been estimated at $80,000.