Witnesses reported hearing unusual noises coming from the aircraft as it lifted off after refueling, said Dennis Hogenson, deputy regional chief of the Western Pacific Region for the National Transportation Safety Board.
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.