Investigators headed Saturday to the crash site to recover the plane's data recorders and the man's remains as part of a criminal probe, authorities said.
The sheriff's department said "preliminary" information was that the plane's pilot was "a mechanic from unknown airlines" who "was doing stunts in air" or lacked flying skills.
The Pierce County Sheriff's Office said the man, who has been identified as Richard Russell, was 29 years old and "suicidal".
The plane wasn't even scheduled to fly at the time of the incident, he said. Horizon Air is a division of Alaskan that flies shorter routes in the western US.
The family of Richard Russell, the airport ground services worker who stole a plane from Sea-Tac International Airport and crashed it after a dramatic flight, has released a statement praising his compassion and expressing "complete shock" over what happened.
Authorities say he commandeered an empty Bombardier Q400, 32.61-metre long turboprop aircraft on Friday (Aug 10) night from a maintenance area at Seattle-Tacoma International Airport. "We are giving those investigators our full support and cooperation".
While in the air, Russell joked for almost 20 minutes with the air traffic control agent, but admitted he had "a few screws loose" and called himself "just a broken guy".
"I watched probably 18 planes land and line up across from us because they were getting everyone on the ground and out of the air patterns", said Leddy.
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Man who stole plane was amusing , hardworking, former colleague recalls
He went through various background checks to get clearance to be in the secured area, authorities said. "So many bags", he said. Pierce County Sheriff Paul Pastor said: "This man did something foolish and may well have paid with his life".
Alaska Air Group CEO Brad Tilden also issued a statement. His work as a ground agent included handling baggage, tidying airplanes and operating tow vehicles to move aircraft, and he was "fully credentialed and had access to that area legitimately". "It's going to disappoint them to hear that I did this", he said.
Mr Southers said the man could have caused mass destruction.
At one point, he said: "Hey do you think if I land this successfully Alaska will give me a job as a pilot?" Got a few screws loose.
Video showed fiery flames amid trees on the island, which is sparsely populated and only accessible by ferry.
No passengers were on board and normal operations have resumed at the airfield, the airport said on Twitter. The crash sparked a 2-acre wildfire.
But concern about the mental state of aviation workers has grown in recent years, analyst say, particularly after the 2015 crash of Germanwings 9525 flight in France, when a pilot deliberately steered his plane into a mountainside, killing 144 passengers and five crew members.
President Donald Trump was briefed and the White House praised authorities´ quick response to the crisis.
The FBI is leading the probe, which also includes the Federal Aviation Administration and the National Transportation Safety Board.
"It was unfathomable. It was something out of a movie", he told the newspaper.