An apparently suicidal mechanic stole an empty passenger plane from the Seattle-Tacoma airport in the United States (US) late on Friday, took it for a brief flight then crashed it in an incident that officials said was unrelated to terrorism.

Video taken by a bystander showed the passenger airplane making an unlikely upside-down aerial loop, then flying low over Puget Sound before crashing into the sparsely populated Ketron Island in the northwestern US state of Washington.

Two military F-15s were scrambled to chase the stolen plane, but local officials said the jets “were not involved in the crash".

The crash sparked a fire in the dense forest. Flames lit up the night as they spread from the burning wreckage to nearby trees.

An airline employee “conducted an unauthorised takeoff” of an airplane carrying no passengers, then “crashed in south Puget Sound,” Sea-Tac Airport officials said on Twitter.

The stolen plane was a twin-engine turboprop Q400 airplane belonging to its sister carrier Horizon Air, Alaska Airlines said on Twitter. It normally carries 76 passengers.

“This is not a terrorist incident,” Ed Troyer with the Pierce County Sheriff's office wrote on Twitter. “Confirmed info... this is a single suicide male. We know who he is. No others involved,” Troyer wrote

The suspect was identified as a 29-year-old airline mechanic and Pierce county resident who “acted alone”, Troyer added, confirming that there were “no passengers on the plane” when it crashed.

The sheriff's office also said that the F-15s, which were scrambled out of Portland, in the nearby state of Oregon, arrived minutes after the plane was stolen and kept the aircraft “out of harm's way and people on ground safe".

Horizon Air Chief Executive Officer Constance Van Muehlen issued a video statement describing the incident.

“We believe it (the plane) was taken by a single Horizon Air employee and no other passengers or crew were on board,” she said. “Our hearts are with the family of the individual aboard as well as all of our Alaska Air and Horizon Air employees.”

"Bizarre" incident filmed

John Wauldron, who took a dramatic video of the stolen plane flying in a loop, told CNN that he was out for an evening stroll when he saw the two jet fighters following the turboprop airplane.

His first thought was that they were practicing for an air show.

“So, I started to capture video, just because I thought it was, kind of bizarre,” he told CNN.

Wauldron said it seemed that the jets were chasing down the airplane. “I thought this is really odd. Kept the video rolling.”

Then the passenger plane pilot “did a complete loop ... I couldn't believe he recovered".

He estimates that the plane at its lowest point “was no less than 100 feet above the water". Then the pilot “pulled -- pretty much straight up. And kind of at an angle. And almost stalled the aircraft. Somehow he got it leveled back off. And then made his way down toward the island.”

Wauldron said that he was prepared to “run and take cover". He briefly turned away, then turned back and saw the explosion as the plane crashed.

“Saw a bright, pinpoint area of flame. And the smoke. I thought, oh, my god. I think he just crashed."