Investigators have been trying to figure out how a "suicidal" airline employee stole an empty Horizon Air turboprop plane, took off from Seattle's Sea-Tac International Airport and crashed into a small island in the Puget Sound after being chased by military jets that were quickly scrambled to intercept the aircraft.
The Friday night crash happened because the 29-year-old man was "doing stunts in air or lack of flying skills", the Pierce County Sheriff's Department said.
The man, who was believed killed, wasn't immediately identified.
He was suicidal and there was no connection to terrorism, Ed Troyer, a spokesman for the sheriff's department, said on Twitter.
Earlier video showed the Horizon Air Q400 doing large loops and other dangerous manoeuvres as the sun set on the Puget Sound. There were no passengers aboard.
The plane - a Bombardier Q400 turboprop capable of carrying 76 passengers - was pursued by military aircraft before it crashed on Ketron Island, southwest of Tacoma, Washington.
In a recording of his conversation with air traffic controllers, the man could be heard saying he had a "few screws loose" and "never really knew it until now".
"Until the FBI has the opportunity to get better background on the person, find out what motive they had, it's a little too early to make a determination on what the objective was," said Debra Eckrote of the National Transportation Safety Board.
Where to find help and support:
Need to Talk? - Call or text 1737
Lifeline - 0800 543 354 or (09) 5222 999 within Auckland
Samaritans - 0800 726 666
Depression Helpline - 0800 111 757
- Suicide Crisis Helpline - 0508 828 865 (0508 TAUTOKO)
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