A mechanic stole a propeller airplane at Seattle-Tacoma International Airport on Friday before crashing it, authorities said.
The incident was not considered terrorism, the FBI's Seattle office tweeted.
And there was no indication that the incident was an act of terror, Pierce County Sheriff Paul Pastor told KIRO7 news station in Seattle. Early reports indicate that an airport employee stole the jet, causing fighter jets to be deployed and all ground traffic was halted.
Sea-Tac operations have resumed as normal, airport officials said.
The sheriff's department said "preliminary" information was that the plane's pilot was "a mechanic from unknown airlines" who "was doing stunts in air" or lacked flying skills.
Scott Adams, a battalion chief with West Pierce Fire and Rescue, said about 10:30 p.m. the crash had started a grass fire, and that two fire engines, a medic unit and a command unit were at the site, along with firefighting staff from Joint Base Lewis-McChord.
Gov. Jay Inslee thanked the Air National Guard from Washington and OR for scrambling jets and said in a statement "there are still a lot of unknowns surrounding tonight's tragic incident".
Witnesses reported seeing the plane being chased by military aircraft before it crashed on Ketron Island, southwest of Tacoma, Washington.More news: Boris Johnson to face inquiry over burka comments
Two flights were delayed at Spokane International Airport because of the Sea-Tac delay.
The air-traffic controller responded calmly, seeming not to want to upset Rich as the conversation continued and he tried to coax Rich into landing somewhere. Wrote one man on Twitter, "As terrible as this is, I have to say this: an aircraft mech pulled off a barrell roll in a twin engine turbo prop commuter plane".
Jenkins described the island as remote, dark and sparsely populated.
After the plane approached Ketron Island, Waldron saw a thick column of smoke followed by a loud explosion. We're trying to get accurate information about what is actually going on.
He told authorities on the ground he "would like to apologize" to people who cared about him.
"We're working to confirm who was on board, we believe there were no guests or crew on board other than the person operating the plane", the company said. We believe there are no passengers on board.
The plane, a 76-seater Q400 turboprop aircraft, belonged to Alaska Airlines.