The US Federal Bureau of Investigation has taken charge of the inquiry into a stolen Horizon Air turboprop that crashed on 10 August after a short flight during which the aircraft performed aerobatic-like manoeuvres while pursued by air force fighter jets.
A 29-year-old Horizon employee who apparently lacked a pilot's license stole the Bombardier Q400 (registration N449QX) and died when the aircraft crashed in a wooded area in Pierce County in Washington state, officials say.
Officials do not believe anyone else was injured, although they stress the investigation is ongoing.
"We are diligently investigating this matter… I have dozens of personnel out at the crash site," FBI special agent Jay Tabb tells reporters during a press conference on 11 August. "At this time we believe he was the only one in the aircraft."
Horizon chief executive Gary Beck says: "A Horizon Air ground service agent departed in a Horizon Air Q400 without clearance... The plane was taken from a maintenance position and was not scheduled for passenger flight."
Horizon is the regional subsidiary of Alaska Air Group, operating flights marketed by Alaska Airlines.
"All of us at Alaska and Horizon are deeply saddened by last night's unauthorised flight… that resulted in a loss of life," says Alaska chief executive Brad Tilden. "We are working closely with the authorities."
Prior to crashing, the ground service agent exchanged bizarre radio messages with officials on the ground.
"I've got a lot of people that care about me and it's going to disappoint them… that I did this," he says, according to recordings available at www.liveatc.net. "I would like to apologise. Just a broken guy. I have a few screws loose."
People on the ground shot video apparently showing the Q400 performing rolls and descending steeply to low altitude above water. Videos also show fighter jets tailing the Q400.
"There were some manoeuvres that were [performed] that were incredible," an Alaska official tells reporters.
The FBI's Seattle office confirms the federal agency is leading the inquiry. The US National Transportation Safety Board is helping the FBI and will examine the Q400's cockpit voice and data recorders at labs in Washington DC, it says.
The event was "not a terrorist incident", but rather the work of "a single suicide male", says the Pierce County Sheriff's office in tweets. The sheriff's office identifies the pilot as a 29-year-old Pierce County resident.
Bombardier manufactured the Q400 in 2012 and it had accumulated 14,200h and 15,000 cycles as of April, according to Flight Fleets Analyzer.
The ground service agent stole the aircraft from where it was parked in a cargo area at the north end of the airport, Tilden tells reporters. Like other ground service agents, the employee had access to secure areas of the airport where aircraft is parked.
The employee had worked earlier in the day and had finished his shift, and was still in uniform. He used a pushback tractor to rotate the aircraft 180˚ before getting into the aircraft and taxiing out, say Alaska executives.
It was not immediately clear how the aircraft made it to the runway without being stopped. Alaska executives declined to speculate, saying the investigation is ongoing.
Horizon hired the employee in February 2015 and had vetted him via background checks, the airline says. The employee's job included loading and unloading aircraft, and working aircraft tugs, officials say.
The aircraft took off without clearance from Seattle Tacoma International airport at about 19:32 local time, and Alaska lost contact with the Q400 at about 20:47.
The Federal Aviation Administration notified the North American Aerospace Defense Command (NORAD) of the incident, and NORAD quickly dispatched two Boeing F-15C fighters from Portland to intercept the Q400.
"The fighters were directed to fly supersonic to expedite the intercept," NORAD says in a statement.
Fighters intercepted the Q400 south of the airport, near McCord Air Force Base, NORAD says. The military pilots tried to direct the Q400 pilot to fly toward the Pacific Ocean.
"NORAD fighters did not fire upon the aircraft," NORAD adds.
Officials communicated with the Horizon employee via radio for at least 20min during the flight, trying to convince him to make a controlled landing or ditch the aircraft.
"Can this thing do a back flip, do you think?" the employee says. "I'm going to try to do a barrel roll, and if that goes good I'm just going to nose it down and call it a night."
"Before you do that, let's think about this," controllers respond.
The pilot then talks about the beauty of the nearby Olympic Mountains and says he expected to have a "moment of serenity".
Eventually, he says: "I feel like one of my engines is going out".
The aircraft ultimately crashed about 25nm (46kmn) southwest of the airport on Ketron Island in the southern Puget Sound, officials say.
"It is highly fragmented. The wings are off. The fuselage is... upside down," NTSB regional chief Debra Eckrote tells reporters on 11 August. Work is underway to locate the aircraft's cockpit voice and flight data recorders.
"Last night there was a fire, so they weren't able to identify everything," she adds. "The terrain is thick underbrush… and heavily wooded."