They also said the aircraft rotated counterclockwise before it crashed near the Seattle Center campus, which is home to the Space Needle, restaurants and performing arts centers.
Bo Bain, an excavation foreman at a nearby construction project, watched the helicopter land as usual, one of many flights he has watched come and go in recent months. But he said something sounded different when the aircraft left the helipad Tuesday morning.
"It pitched sideways. It was off balance, and you could tell right away something wasn't right," Bain said. "The helicopter was struggling to stay up. It spun around, hit the top of the tree and landed on the street."
Seconds later, he said: "It was just a fireball. The whole thing burst into flames. I saw people running from their cars."
Hogenson said a preliminary report on the crash is expected in five days, followed by a fuller report with a probable cause in up to a year.
KOMO identified the pilot as Gary Pfitzner, of Issaquah. The other man killed in the crash was Bill Strothman, a former longtime KOMO photographer. Both men were working for Cahokia, Ill.-based Helicopters Inc., which owned the Eurocopter AS350 helicopter. The aircraft was leased jointly by KOMO and KING-TV.
The helicopter was a temporary replacement for one that's in the shop for an upgrade, KOMO reported.
Firefighters who arrived at the scene before 8 a.m. found a "huge black cloud of smoke" and two cars and a pickup truck engulfed in flames, Seattle Fire Department spokesman Kyle Moore said.
Fuel running down the street also was on fire, and crews worked to stop it before it entered the sewer.
An injured man managed to free himself from a burning car and was taken to Harborview Medical Center, Moore said. The man was on fire and KOMO reported that one of its building security guards, Brian Post, ran toward the fire to help.
"I used my hand at first and then his jacket to get the flames out," Post, a former police officer, told the station.
Richard Newman, 38, suffered burns on his lower back and arm, covering as much as 20 percent of his body, hospital spokeswoman Susan Gregg said. He was in serious condition in the intensive care unit and likely will require surgery, she said Tuesday.
Two others who were in vehicles that were struck by the helicopter were uninjured.
KOMO is a block from the Space Needle and is surrounded by high-rise office and apartment buildings. Workers at the station rushed to the window when they heard the crash. KOMO reporters were then in the position of covering their colleagues' deaths.
One of them, Denise Whitaker, said on the street shortly after the crash: "It is a difficult time for all of us this morning."
News anchor Dan Lewis described Strothman as someone "who really knew how his pictures could tell a million words."
"He was just a true gentleman," Lewis said on the air. "We're going to miss you guys."
The Strothman family said in a statement that the former KOMO photographer was a "great man, a kind soul, a devoted husband, a loving father and brother."
Mark Pfitzner said in a statement that his brother Gary loved adventure, to travel and to fly. He was the oldest of seven kids and "took great care of his brothers and sister."
Other cities have experienced helicopter crashes as TV stations rush to cover the news from above major cities.
The last helicopter crash in Seattle was in November 1999, when a KIRO-TV news helicopter collided in midair with another helicopter over Lake Union. There were only minor injuries, after both pilots landed safely at nearby helipads, according to an NTSB report.
Current rules in Seattle allow helipads to be used downtown and in some commercial zones and industrial areas. They can be used only for public service, emergency medical care and for news agencies, mayor's office spokesman Jeff Reading said.
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.
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