The man was a resident of Pierce County, Wash., and "acted alone", the Pierce County Sheriff's Department tweeted, describing him as "suicidal".
The mechanic commandeered the Horizon Air Q400 aircraft from Seattle-Tacoma International Airport, and the crash occurred because he was "doing stunts in [the] air or [a] lack of flying skills", the sheriff's department said on Twitter on Friday. The aircraft then crashed into Ketron Island, 40 km to the southwest of the Seattle/Tacoma Airport.
It also said "this was not a terrorist incident". Witnesses said and Alaska Airlines plane is going down. "There are still a lot of unknowns surrounding tonight's tragic incident of a stolen Horizon Air plane from SeaTac Airport".
In a partial recording made by website The Air Current, the pilot tells air traffic controllers: "there's something wrong, I feel like one of my engines is going out or something".
Released radio conversation between the man and air traffic controllers goes on for more than a half hour.
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"Alright", the man in the airplane responds. I don't know! I don't want to. "I was kind of hoping that was gonna be it". Lines were modest and most flights were on time. "We are working background on him now", the sheriff's department tweeted. It did not elaborate.
According to the eyewitnesses, F-15 military jets were rushed to intercept the stolen plane soon after its unauthorised take-off from the airport.
"It's super scary when you think about how fast those planes are moving, the distance from Ketron Island to these houses in minuscule".
Only the unidentified pilot was on the plane, according to authorities.
As Mr Waldron was preparing to "run and take cover", he says he looked back and "saw a bright, pinpoint area of flame".
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Waldron said it seemed that the jets were chasing down the airplane.
"We looked across the water there's this huge fireball coming up off the west coast of Ketron Island, followed by smoke".
"Everyone was literally frozen in place muttering about what possibly happened", he said.
In a series of tweets, he said F15s were scrambled out of Portland and arrived within a few minutes of the theft of plane.
"My husband and I are recently former Boeing employees and we were wondering why a jet was near the passenger airplane", she said.
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