The hijacker of an empty Horizon Air turboprop plane in Seattle had clearance to be among aircraft but did not hold a pilot’s licence, authorities said.

The 29-year-old man used a machine called a pushback tractor to first manoeuvre the aircraft so he could board and then take off from Sea-Tac International Airport on Friday evening.

He didn't have a pilot's license and authorities say it's unclear how he gained the skills to do loops in the aircraft before crashing into a small island about an hour after taking off.

Authorities said the man went through various background checks to get clearance to be in the secured area.

The plane broke into pieces when it crashed into an area of thick brush, but investigators remain hopeful they will be able to recover the aircraft’s data recorders.

Authorities initially said the man was a mechanic but Alaska Airlines later said he was an employee who helps direct aircraft to gates and de-ice planes.

He told air traffic controllers that he was a "broken guy" but also joked about whether the airline would hire him as a pilot if he landed safely.

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.

He told them: "Those guys will rough me up if I try and land there."

The empty passenger airplane, stolen from the Seattle-Tacoma airport, making an unlikely upside-down aerial loop, then flying low over Puget Sound before crashing into the sparsely populated Ketron Island (Getty)

During another part of the exchange, the man said he was concerned he was going to run low on fuel.

Later, he said he's "got a lot of people that care about me."

He said he didn't want to disappoint them but that he was "just a broken guy, got a dew screws loose, I guess."

Pierce County Sheriff Paul Pastor said the man "did something foolish and may well have paid with his life."

The FBI is looking into the man's background and trying to determine his motive.

Alaska Air Group CEO Brad Tilden said in a statement early Saturday morning that the airline was "working to find out everything we possibly can about what happened."

The airline was coordinating with the Federal Aviation Administration, the FBI and the National Transportation Safety Board, he said.