Three vehicles on the street caught fire as the helicopter crashed right outside KOMO's studios.
A witness told KOMO the engine of the helicopter "sounded extra heavy" as it started up, and he heard an explosion seconds later.
Firefighters arrived to find a huge plume of thick black smoke, said fire department spokesman Kyle Moore.
Aviation fuel leaked from the wreckage and caught fire, helping firefighters' efforts to keep it from contaminating the city's sewer system.
Mr Moore said investigators had no indication the helicopter struck a building.