North American Aerospace Defense Command scrambled fighter jets to accompany the plane, which was steered away from Seattle and Tacoma by air traffic control, on its hour-long flight, according to the source. The Pierce County Sheriff's Department said the two F-15s were not involved in the crash.
The crash happened on Ketron Island, which is southwest of Tacoma, in south Puget Sound. The small island has only about 20 year-round residents.
The employee who stole the plane was 29 years old, from Pierce County and called "suicidal" by the sheriff's department. Horizon Air COO Constance von Muehlen confirmed the man was an employee of the company.
The sheriff said this was not a terrorist incident.
"Most terrorists don't do loops over the water," Sheriff Paul Pastor said. "This might have been a joyride gone terribly wrong."
The Pierce County Sheriff's Department has a number of vessels on scene, while the U.S. Coast Guard is on scene as well.
The Pierce County Sheriff tweeted, "Stolen horizon airplane crashed into Ketron island. 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."
Alaska Airlines tweeted about an incident involving an "unauthorized take-off of a Horizon Air Q400. They said there were no passengers on board. Horizon Air is operated by Alaska Air Group.
