According to the Pierce County Sheriff's department, the pilot was a "suicidal male".
The hijacker has been described as a 29-year-old mechanic who worked for an "unknown airline".
The plane was taken from a maintenance position and was not scheduled for a passenger flight, said Gary Beck, Horizon Air's president and chief executive officer.
Footage captured by witnesses shows the hijacked Horizon Air Q400 performing stunts over Ketron Island near Seattle.
The airline said it was "working to confirm who was on board" but it was understood that there "no passengers or crew other than the person operating the plane".
Some Seattle-bound flights were diverted to Vancouver.
Horizon Air is part of Alaska Air Group and flies shorter routes throughout the USA west.
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The US Coast Guard said it was aware of the situation happening at SeaTac Airport, NBC News reported.
Communication between the hijacker and Air Traffic Control- "I've got a lot of people that care about me". As of 12:30 a.m., SeaTac tweeted that normal operations had resumed.
"I felt like we were on the ground and we were safe, we weren't in the sky while something was going on", he said after getting off the plane.
The man, who was addressed as "Rich" in audio recordings with air traffic controllers, said he didn't want to land at a nearby military base. "Hopefully it is just a stolen plane with one pilot and no passengers". Witnesses said a. plane is going down. "Ah damn it, I don't know, man". "We frequently get involved with matters related to aircraft so we are poised to act if information develops to suggest a federal criminal nexus".
A video uploaded to social media showed an aircraft matching a Q400's profile flying above a rural area, followed by a fighter jet. No one is departing as they work out an issue.
"The responding fighter pilots flew alongside the aircraft and were ready to do whatever was needed to protect us, but in the end the man flying the stolen plane crashed on Ketron Island. Holy smokes, I've got to stop looking at the fuel because it's going down quick". "I would hope it is for a guy like me".
Royal King told The Seattle Times he was photographing a wedding when he saw the low-flying turboprop being chased by to F-15s. "I'm a broken guy who had a few screws loose".