A team from Illinois-based Helicopters Inc., which owned the aircraft, also is in the state assisting with the probe, Dennis Hogenson with the National Transportation Safety Board said Wednesday.
Investigators are poring over pilot, maintenance and company records, and they will recreate the crash scene to look for anomalies, he said. Wreckage from the helicopter has been moved to a hangar in Auburn, about 30 miles south of Seattle.
It may be months before federal investigators know what caused the KOMO-TV news chopper to hit the pavement and burst into flames Tuesday, setting three vehicles ablaze and spewing burning fuel down the street.
The KOMO-TV flight was one of many helicopter flights that take off and land in Seattle's downtown. Mayor Ed Murray said officials would review rules for helicopter pads in the city to determine if any changes need to be made.
Witnesses reported hearing unusual noises coming from the aircraft as it lifted off from the helipad on top of Fisher Plaza after refueling, Hogenson said Tuesday. Witnesses also reported seeing the helicopter rotate before it crashed.
"It pitched sideways. It was off balance, and you could tell right away something wasn't right," said Bo Bain, an excavation foreman at a nearby construction project who watched the aircraft take off. "The helicopter was struggling to stay up. It spun around, hit the top of the tree and landed on the street."
Seconds later, "It was just a fireball. The whole thing burst into flames," he said.
Former KOMO photographer Bill Strothman (left) and pilot Gary Pfitzner (right)Mourners on Wednesday left flowers at the crash site to remember former KOMO veteran photographer Bill Strothman and pilot Gary Pfitzner. Both men were working for Helicopters Inc., which owned the Eurocopter AS350 helicopter. The aircraft was leased jointly by KOMO and KING-TV.
Mark Pfitzner told KOMO that his brother, Gary, put himself through flight school, loved to fly and "tried to do his best reporting for people."
News anchor and reporter Molly Shen remembered Strothman as "one of the best storytellers to have ever graced the halls of KOMO."
Richard Newman, 38, who suffered serious burns when the helicopter crashed on his car, was breathing on his own Wednesday, said Susan Gregg, a spokeswoman with Harborview Medical Center. The Seattle man remained in serious condition.
A man and a woman who were in vehicles that were struck by the helicopter were uninjured.
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.