According to the airlines, there were no passengers on board.
SeaTac Airport tweeted, "An airline employee conducted an unauthorized takeoff without passengers at Sea-Tac; aircraft has crashed in south Puget Sound".
After the plane approached Ketron Island, Waldron saw a thick column of smoke followed by a loud explosion.
Seattle-Tacoma International Airport said the person flying the plane was an airline employee. Then he added, "T$3 his is a single suicide male".
Horizon Air chief operating officer Constance von Muehlen said the company believed the mechanic involved was a Horizon employee.
Footage captured by witnesses shows the hijacked Horizon Air Q400 performing stunts over Ketron Island near Seattle.
No passengers were on board, it said.
Some Seattle-bound flights were diverted to Vancouver.
Airline employee steals plane at Sea-Tac Airport, crashes on Puget Sound island
Officials with Alaska Airlines confirmed the plane as a Bombardier Q400 that belongs to Horizon Air. As often happens in situations like these, some information reported early may turn out to be inaccurate.
In the 25 minute-long recording posted to Broadcastify, the pilot can be heard calmly chatting with Air Traffic Control. I don't know! I don't want to.
The airport says normal operations at Sea-Tac have resumed.
Audio captured between the man - who was identified as "Rich" - and air traffic controllers show him asking for help to get the cabin pressurised so he would stop feeling so lightheaded.
Only the unknown pilot was on the plane, according to authorities. "Ah damn it, I don't know, man". I would like to apologize to each and every one of them.
The air traffic controller can also be heard trying to talk him into landing.
The plane belonging to regional carrier Horizon was stolen from SeaTac and crashed into Ketron Island southwest of Tacoma, officials said. "We frequently get involved with matters related to aircraft so we are poised to act if information develops to suggest a federal criminal nexus". Got a few screws loose, I guess. "I was kind of hoping that was gonna be it". An Alaska Airlines Dash 8 Q400 was stolen from its hanger at Seattle Tacoma Airport early in the evening of August 10, and plane took off without permission of tower.
Witnesses report F-15s chasing plane then saw smoke, and loud boom. No one is departing as they work out an issue.
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