Crews responded to the scene, an apartment complex at 3711 164th Street SW, after receiving a report of smoke and flames coming from the building.
Two injured people were found. One was treated at the scene and the other was taken to Providence Hospital in Everett with non-life-threatening injuries.
Witnesses said the fire was intense when crews first arrived.
Firefighters quickly cut holes in the roof to vent the blaze and brought it under control, but smoke continued to pour out for some time afterward.
There was no immediate word on what caused the fire or how many units were damaged.