An airport employee who hijacked an empty commercial plane from Seattle-Tacoma Airport joked with air traffic controllers for more than 20 minutes before crashing it into an island 25 miles away.

The 76-seater Horizon Air turboprop Q400 took off at around 8pm, and two F-15 fighter jets scrambled from Portland 'minutes later' to intercept it, according to Pierce County Sheriff's Office.

Witnesses described seeing the aircraft performing barrel rolls and loop-the-loops as the military planes directed it away from highly-populated areas and towards Ketron Island, where it crashed into a ball of flame.

While still in the air, the pilot was heard telling traffic controllers he was 'just a broken guy' before telling them he was preparing for 'jail time for life'. Police blamed 'doing stunts in the air and a lack of flying skills' for the crash.

The pilot was a 29-year-old from Pierce County, Washington, who was 'suicidal' and acting alone, the Sheriff's Office said. He has not been named, but air traffic controllers addressed him as 'Rich'.

Sheriff Paul Pastor confirmed the incident was 'not terrorist related' and described it as 'a joyride gone terribly wrong'.

In a statement just before midnight, Alaska Airlines said a ground service agent took an out-of-service plane without clearance. Part of a ground service agents job is to direct and de-ice planes, as well as managing luggage.

The aircraft was not scheduled for passenger flights, they added.

Shortly after the plane took off, traffic controllers were heard on an Internet livestream speaking to a man identified as 'Rich'.

'There is the runway just off your right side in about a mile, do you see that?' the traffic controller said.

'Oh those guys will try to rough me up if I try land there…,' Rich replied. 'I think I might mess something up there too. I wouldn't want to do that. Oh they probably have got anti-aircraft.'

'They don't have any of that stuff, we are just trying to find you a place to land safely.'

'Yeah, not quite ready to bring it down just yet, but holy smokes I need to stop looking at the fuel 'cos it's going down quick.'

'OK, Rich, if you could, could you start a left-hand turn and we'll take you down to the south-east.'

'This is probably jail time for life, huh? I would hope it is for a guy like me.' The audio segment then stopped.

Ketron Island, where the plane went down, is a densely wooded area home to 24 people, according to the 2000 census. None of the island's residents were thought to have been harmed.

Questions will now be asked about security at the airport and how an unqualified worker was given access to the plane. It is also unclear how he was able to take off unhindered.

Royal King told The Seattle Times he was photographing a wedding when he saw the low-flying turboprop being chased by to F-15s. He said he didn't see the crash but saw smoke.

'It was unfathomable, it was something out of a movie,' he told the newspaper. 'The smoke lingered. You could still hear the F-15s, which were flying low.'

The airport was shut for around an hour and a half as the incident was dealt with. During this time, flights were suspended and planes already on the runway ordered to stay put.

A Twitter user called 'Victoria' wrote that she was sitting in a plane on the runway awaiting more news from the pilot, who had explained another aircraft had been hijacked.

Around 20 minutes later she tweeted again to say her plane was taxiing off the runway.

Images from the airport terminal showed hundreds of Air Alaska Airlines passengers queuing with their baggage and awaiting information.

Shortly after the plane took off, traffic controllers were heard on an Internet livestream speaking to a man identified as 'Rich'.

'There is the runway just off your right side in about a mile, do you see that?' the traffic controller said.

'Oh those guys will try to rough me up if I try land there…,' Rich replied. 'I think I might mess something up there too. I wouldn't want to do that. Oh they probably have got anti-aircraft.'

'They don't have any of that stuff, we are just trying to find you a place to land safely.'

'Yeah, not quite ready to bring it down just yet, but holy smokes I need to stop looking at the fuel 'cos it's going down quick.'

'OK, Rich, if you could, could you start a left-hand turn and we'll take you down to the south-east.'

'This is probably jail time for life, huh? I would hope it is for a guy like me. '

The audio then stopped.

Seattle-Tacoma (Sea-Tac) airport said: 'An airline employee conducted an unauthorized takeoff without passengers at Sea-Tac; aircraft has crashed in south Puget Sound. Normal operations at Sea-Tac Airport have resumed.'

Alaska Airlines said: 'We are aware of an incident involving an unauthorized take-off of a Horizon Air Q400. We believe there are no passengers on board. More information as we learn more.'

'This is not a terrorist incident. Confirmed info... this is a single suicide male. We know who he is. No others involved,' the Pierce County Sheriff said.

The Federal Aviation Administration said: 'We can't confirm anything at this time. We're trying to get accurate information about what is actually going on. Without confirming anything, a stolen aircraft would be a security issue.

'The FAA is not a security agency, although we work closely with other government agencies on security issues.'

The incident is now being investigated by the FBI and military. It is thought to be the first fatal incident in the US involving a commercial airliner since 9/11.

Horizon Air is part of Alaska Air Group and flies shorter routes throughout the U.S. West.

Sea-Tac is the ninth busiest airport in the US, and flew 46.9 million passengers and more than 425,800 metric tons of air cargo in 2017.