F-15 military jets were scrambled to intercept the aircraft
A plane has crashed after a US airline employee conducted an "unauthorised take-off" in Washington state, officials have said.
Witnesses reported seeing the plane being chased by two F-15 military jets near Seattle-Tacoma International Airport before it crashed near Ketron Island, south west of Tacoma.
The Pierce County Sheriff's Office said on Twitter that preliminary information suggested that a mechanic had stolen the aircraft.
The suspect was identified as a 29-year-old Pierce County resident and the sheriff's office said he was an employee "doing stunts in air or lack of flying skills".
It added: "This is not a terrorist incident. Confirmed info.. this is a single suicide male. We know who he is. No others involved."
The F-15s had been scrambled from Portland and had "made it within a few minutes of theft of plane", the sheriff's office said.
Officers were conducting a background investigation on the man, who they did not identify but who was identified as "Rich" in audio recordings.
They added that the man could be heard on audio recordings telling air traffic controllers that he's "got a lot of people that care about me" and that he was "just a broken guy".
The US Coast Guard said it has sent a 45ft vessel to the crash scene after reports of a large plume of smoke in the air.
Leah Morse, who watched the plane flying over her house tailed by the jets, said: "I was outside watering and heard the jets, saw the plane coming, and something told me to get my phone and record.
"Felt it in my gut something was wrong."
Ms Morse said her mother, who lives near the crash site, had said eight minutes later that her whole house shook.
"We saw the jets circle back after and she texted that there was no plane."
Footage was also posted on Twitter.
Some dude stole a plane from #Seatac (Allegedly), did a loop-the-loop, ALMOST crashed into #ChambersBay, then crossed in front of our party, chased by fighter jets and subsequently crashed. Weird times. pic.twitter.com/Ra4LcIhwfU— bmbdgty (@drbmbdgty) August 11, 2018
The F-15s were not involved in the crash, it was reported.
Horizon Air chief operating officer Constance von Muehlen confirmed one of its employees had made the "unauthorised take-off" at around 8.00pm local time.
She confirmed no other crew or passengers had been on the 76-seater Q400.
"Our hearts are with the family of the individual aboard as well as all our Alaska Air and Horizon Air employees," she added.
Horizon Air is part of Alaska Air Group and flies shorter routes throughout the US west.
Seattle-Tacoma International Airport officials said normal flight operations have resumed.
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