A plane has taken off at Seattle-Tacoma International Airport without authority and crashed into south Puget Sound after F-15 fighter jets were scrambled to pursue the aircraft.
Alaska Airlines confirmed a Horizon Airlines Q400 Turboprop made an unauthorised take-off from the airport.
No passengers were on board the plane, the airline said.
Sheriff's officials say the man who stole the Alaska Airlines plane was "suicidal" and there is no connection to terrorism.
Ed Troyer, a spokesman for the Pierce County Sheriff's Department, said on Twitter that a 29-year-old airline mechanic stole the plane.
Witnesses reported seeing the plane being chased by military aircraft before it crashed near Ketron Island.
The sheriff's department said they were working to conduct a background investigation on the man, whose name was not immediately released.
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 is "just a broken guy."
Excerpts from the audio reveal the air traffic controller urged the man to land the plane.
"We're just trying to find a place where you can land the plane safely," the controller says. The man replies: "Yeah, not quite ready to bring it down just yet. Holy smokes, I've got to stop looking at the fuel because it's going down quick."
Seattle-Tacoma International Airport said the person flying the plane was an airline employee.
A plume of smoke has been seen near Ketron Island, Washington, the US Coastguard reported.
Local KIRO-7 Seattle reporter Gary Horcher said on social media indications were the suspect was speaking to air traffic controllers.
The Federal Aviation Administration issued a message to media:
"We can't confirm anything at this time. We're trying to get accurate information about what is actually going on. Without confirming anything, a stolen aircraft would be a security issue.
"The FAA is not a security agency, although we work closely with other government agencies on security issues."
Flights departing SeaTac have now resumed.
Morgan Chesky, a reporter for KOMO News, wrote on Twitter, "Per JBLM source: After being contacted by F-15's, stolen plane began to nose dive, crashing in north Pierce County."
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