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The chopper was taking off from the KOMO-TV station when it went down on Broad Street and hit three vehicles, starting them on fire and spewing burning fuel down the street.
Kristopher Reynolds, a contractor working nearby, saw the wreck. He said the helicopter lifted about 5 feet and was about to clear a building when it tilted. It looked like it was trying to correct itself when it took dive downward.
"Next thing I know, it went into a ball of flames," he said.
When firefighters arrived, they found the helicopter, two cars and a pickup truck on fire, along with a huge cloud smoke, fire department spokesman Kyle Moore said.
"Not only were the cars on fire, the fuel running down the street was on fire," he told reporters at the scene.
Firefighters stopped the burning fuel from entering the sewer.
The injured man managed to free himself from one of the cars and was taken to Harborview Medical Center with burns on more than 50 percent of body, the Seattle Fire Department said. Hospital spokeswoman Susan Gregg said the facility had received no other victims from the crash.
A woman from one of the other burned cars went to a police station and talked to officers. The man from the pickup truck walked off. Fire investigators want to talk to him, Moore said.
The two who were killed remained in the wreckage until investigators from the Federal Aviation Administration and National Transportation Safety Board arrived, Moore said.
An hour after the crash, firefighters had put out the flames and were cleaning up the spilled fuel, which left a strong smell in the area. Only the tail of the helicopter could be identified among the burned metal on the street next to the Seattle Center.
Workers at KOMO rushed to the window when they heard the crash. Reporters with the station were then in the position of covering the deaths of colleagues.
"We mourn the loss of a couple of our co-workers today," KOMO-TV anchor Dan Lewis said on the air. "It's so difficult for us to look at this scene, of the wreckage down there."
On the street, reporter Denise Whitaker said, "It is definitely a tragic scene down here. It is a difficult time for all of us this morning."
The victims' names have not yet been released.
The helicopter was a Eurocopter AS350, said FAA spokesman Allen Kinetzer.
It was departing from the downtown helipad when it crashed and burned under unknown circumstances, he said. The station said the chopper might have hit the side of a building before it went down.
The FAA is investigating but the NTSB is the lead agency, Kinetzer said.
Lewis said it wasn't the regular KOMO helicopter but a temporary replacement for one that's in the shop for an upgrade.
The crash site also was near the EMP Museum, the music and culture museum created by Microsoft co-founder Paul Allen. The Seattle Center is popular with tourists and locals, and is the site of many music and cultural festivals and sporting activities.
Other cities have experienced helicopter crashes as TV stations rush to cover the news from above major cities.
Two news helicopters collided in midair in Phoenix in 2007 as the aircraft covered a police chase, sending fiery wreckage plummeting onto a park. Four people in the helicopters were killed.
The crash prompted changes at the stations in how they operated their helicopter crews.