The sheriff said the pilot appeared to have acted alone and was likely killed in the crash.
Deputies said they know who the 29-year-old man is and they described him as suicidal. Early reports indicate that an airport employee stole the jet, causing fighter jets to be deployed and all ground traffic was halted.
Authorities in Seattle, Washington, are responding to an incident involving an aircraft that took off without permission on Friday night.
Normal operations at the airport were interrupted for a time by the unauthorized takeoff but have resumed, the airport said in its tweet. The Federal Aviation Administration reported about 9:30 p.m. that a plane crashed.
Fighter pilots from the Air National Guard of Washington and OR flew alongside the plane, according to an emailed statement from Gov. Jay Inslee.
Alaska Airlines confirmed that a Horizon Air Q400 was involved in an "unauthorized take-off" in a tweet.
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This individual who took the aircraft, who has not yet been positively identified until remains are examined, is believed to have been the only person on the plane when the plane was taken from a maintenance position at Sea-Tac.
"Congratulations, you did that, now let's try to land that airplane safely and not hurt anybody on the ground", an air traffic controller said. The man's condition wasn't immediately known.
Later he says he's going to land it in a "safe manner" but then asks if the plane can do a barrel roll.
"We looked across the water there's this huge fireball coming up off the west coast of Ketron Island, followed by smoke".
No passengers were on board, it said. More information as we learn more'.
In the audio, the man flying the plane can be heard apologising to the people who care about him before referring to himself as a "broken guy" with "a few screws loose".
A Q400 Turboprop, like that stolen from Sea-Tac International Airport. "The FAA is not a security agency, although we work closely with other government agencies on security issues".