OLYMPIA, Wash: Investigators are piecing together how an airline ground agent stole an empty commercial airplane, took off from Sea-Tac International Airport and crashed into a small island in the Puget Sound after being chased by military jets that were quickly scrambled to intercept the aircraft.

Officials said Saturday that the man was a 3.5-year Horizon Airlines employee and had clearance to be among aircraft, but that to their knowledge, he wasn't a licensed pilot.

The 29-year-old used a machine called a pushback tractor to first manoeuvre the aircraft, which was in a maintenance area, so he could board and then take off Friday evening, authorities said.

A US official briefed on the matter told The Associated Press the man was Richard Russell. The official wasn't authorised to discuss the matter and spoke on the condition of anonymity.

Video showed the Horizon Air Q400, a turboprop plane that seats 76 people, doing large loops and other dangerous manoeuvres as the sun set on Puget Sound.

Two F-15C aircraft were scrambled from Portland and pursued the plane but authorities say they didn't fire on it before it crashed on tiny Ketron Island, southwest of Tacoma, Washington. Video showed flames amid trees on the island, which is sparsely populated and only accessible by ferry. No structures on the ground were damaged by the plane, which sparked a small wildfire.

"It is highly fragmented,'' Debra Eckrote, the Western Pacific regional chief for the National Transportation Safety Board, said of the plane. "The wings are off, the fuselage is, I think, kind of positioned upside down.''

Investigators expect they will be able to recover both the cockpit voice recorder and the event data recorder from the plane.

Russell is presumed to have died in the crash.

He could be heard on audio recordings telling air traffic controllers that he is "just a broken guy.''

An air traffic controller tried to convince him to land the airplane.

"There is a runway just off to your right side in about a mile,'' the controller says, referring to an airfield at Joint Base Lewis-McChord.

"Oh man. Those guys will rough me up if I try and land there,'' the man responded, later adding "This is probably jail time for life, huh?''

Later the man said: "I've got a lot of people that care about me. It's going to disappoint them to hear that I did this ... Just a broken guy, got a few screws loose, I guess.''

Russell's family said in a statement that they are stunned and heartbroken. They referenced the recordings of him talking to air traffic controllers and said and that it's clear Russell, who went by the nickname "Beebo,'' didn't intend to harm anyone and "he was right in saying that there are so many people who loved him.''

Horizon Air is part of Alaska Air Group and flies shorter routes throughout the US West. The Q400 is a turboprop aircraft with 76 seats.