The 76-seater aircraft took off without authorization Friday night with a Horizon Air employee at the controls, airport officials tweeted.
The unidentified 29-year-old mechanic was pronounced dead after the crash, Pierce County Sheriff Paul Pastor said.
The incident was not considered terrorism, sheriff’s spokesman Ed Troyer said.
The mechanic “was doing stunts in air or lack of flying skills caused crash into Island,” the sheriff’s department tweeted.
Two F-15 jets chased the plane
Within minutes of the plane’s takeoff, the military scrambled two F-15 jets from Portland to follow the plane, authorities said. They pursued the aircraft before it went down on Ketron Island between Tacoma and Olympia.
“Pilots kept plane out of harms way and people on ground safe,” the sheriff’s office tweeted. It said the jets were not involved in the crash.
Witnesses described the surreal scene after the plane left the airport. John Waldron was walking along a trail near the bay when he noticed the two military jets following another plane.
“I was getting prepared to run just in case,” he said.
After the plane approached Ketron Island, Waldron saw a thick column of smoke followed by a loud explosion.
“Everyone was literally frozen in place muttering about what possibly happened,” he said.
Kethleen Reichel said she watched the planes pass by for about two minutes.
“My husband and I are recently former Boeing employees and we were wondering why a jet was near the passenger airplane,” she said. “We heard the jet because our windows were down while we were waiting in the ferry line to go to Anderson Island which is adjacent to Ketron Island.”
Airport was briefly shut down
Alaska Airlines said the Horizon Air Q400 turboprop aircraft was involved in an unauthorized takeoff around 8 p.m. local time.
“Our hearts are with the families of the individual aboard as well as all of our Alaska Air and Horizon Air employees. We will provide more information as it becomes available,” the airline said.
The mechanic was an employee of Horizon Air, a regional airline based in Seattle-Tacoma airport. It’s the sister carrier of Alaska Airlines.
The plane was taken from a maintenance position and was not scheduled for a passenger flight, the airline said. It crashed about an hour later, and did not hit any ground structures.
Normal operations at the airport were interrupted briefly but have resumed, the airport said.
CNN’s Paul Murphy contributed to this report.