Puget Sound Energy spokeswoman Allison Stanford says crews have responded to the scene.
Fire Department spokesman Kyle Moore told KOMO Radio a man and woman in their 50s suffered burns in the fire and were taken to Harborview Medical Center. One of them has life-threatening injuries and the other is serious.
The blast about 6 a.m. Monday was heard miles away. It blew out windows and rained debris within a one-black area.
Firefighters worked to flames from spreading to nearby homes. The fire was visible from Interstate 5.
A natural gas leak was reported Sunday in the neighborhood.