AN airline worker stole an aeroplane from Seattle-Tacoma International Airport before crashing into a nearby island.
Two US military jets were scrambled to chase the rogue pilot, who slammed the plane onto the sparsely populated island.
There was no indication that the incident was an act of terror, the local Pierce County Sheriff's Department has said.
The sheriff suggested the pilot may have been suicidal and said earlier that the man was a airline mechanic, but the airline said the employee was a ground service agent.
The 29-year-old is believed to have acted alone and to have been killed in the crash.
The sheriff tweeted: ”Was doing stunts in air or lack of flying skills caused crash into Island.”
Two F-15s fighter jets pursued the plane but were not involved in the crash at Ketron Island, according to local TV station KIRO7 reported.
Constance von Muehlen, chief operating officer of Horizon Air, a sister carrier of Alaska Airlines, said that at about 8 p.m. an employee of the airline undertook an unauthorised take-off of a Q400 twin-engine turboprop airplane.
It is unclear how the employee was able to take off without authorisation.
An Alaska Air spokesman said they did not have information about how the incident occurred.
"Told F-15s made it within a few minutes of theft of plane. Pilots kept plane out of harm's way and people on ground safe," the sheriff tweeted.
Video footage on social media showed a large plane flying above the Seattle area with an F-15 following it.
In a statement, the airport said: “An airline employee conducted an unauthorized takeoff without passengers at Sea-Tac; aircraft has crashed in south Puget Sound”
Earlier, the local Sherriff’s office suggested the pilot of the stolen plane was a mechanic or other person with minimal flying experience.
Pierce County Sheriff confirmed on Twitter: “Preliminary info is that a mechanic from unknown airlines stole plane. Was doing stunts in air or lack of flying skills caused crash into Island.”