Two F-15 fighter jets were scrambled on Friday evening after the unauthorised take-off on an aircraft from a Seattle airport prompted a major security alert.
There were no passengers on board when it crashed about 30 miles later, according to local police, who said they knew the identity of the 29-year-old mechanic at the controls and believed him to be suicidal.
Flights were halted in and out of Seattle’s international airport. Passengers on airliners took to social media to relay messages from air crew as they were told their flights would not be taking off.
Alaska Air said a Q400 twin-engined plane, operated by its subsidiary Horizon, was involved.
The unauthorised pilot could be earlier heard talking to air traffic control, in communications relayed on aviation websites, reporting that he believed one of his engines had failed.
In response, he was told to stay low and over water.
Witnesses described seeing the plane nose dive shortly after the F15s arrived in the vicinity.
News crews following the story said they had spotted smoke coming from an island in southern Puget Sound just off the shoreline of Seattle.
Law enforcement officers then tweeted that the plane had crashed and that they believed the pilot was suicidal.
The US Coast Guard was sending a 45-foot vessel to the crash scene after witnesses reported seeing a large plume of smoke in the air, according to Petty Officer.
Some flights resumed at Seattle-Tacoma International Airport later in the evening, although west-bound departures remained on the ground.