The worker banged on the plane for help shortly after takeoff on Monday from Seattle. Pilots heard the noise and quickly returned to the airport. The worker was not injured.
The Federal Aviation Administration was investigating, but few new details emerged Tuesday about the bizarre incident.
Alaska Airlines said the leader of a baggage-loading crew noticed the worker was missing and tried to call and text him before concluding that he had gone home when his 9 1/2-hour shift ended.
Safety experts say the crew should not have closed the cargo doors of Flight 448 to Los Angeles until they had accounted for the missing worker.
"This is a 'huh?' moment," said Thomas Anthony, director of the aviation security program at the University of Southern California and a former FAA official.
"That supervisor said, 'Huh, I wonder where Louie is?' The 'huh' is a yellow light that you need to pay attention to," he said. "The worst thing you can do is just say, 'It's probably nothing.'"
Anthony said airports are responsible for screening workers with access to planes while airlines are responsible for the security of an aircraft.
A U.S. aviation official said there is no legal requirement that airline crews check a cargo hold before every flight. The person spoke on condition of anonymity because airline security programs contain sensitive information and are not public documents.
Incidents like Monday's are rare but have occurred.
— In 2011, a US Airways bag handler was accidentally locked inside the cargo hold of a plane at Reagan National Airport. A passenger heard the man banging on the underside of the floor and alerted a flight attendant.
— In 2009, a bag handler for JetBlue flew from New York to Boston in the cargo hold. The 21-year-old said he fell asleep and panicked when he realized the plane was in flight. He used his cellphone to call the airline during the flight.
— In 2005, a bag handler at LaGuardia Airport in New York fell asleep in the belly of a Spirit Airlines plane and woke up in Detroit.
Alaska Airlines said the baggage handler in Monday's incident was in a pressurized, temperature-controlled part of the cargo hold.
That's much safer than the aircraft wheel wells, where stowaways sometimes hide with no protection from the thin air and extreme cold at high cruising altitudes. Still, a 15-year-old stowaway survived a flight in a wheel well last year from California to Hawaii.
The Seattle worker put in a long day before dozing off in the cargo bay.
Alaska Airlines said in a statement that he was part of a four-person team loading baggage onto the flight and was scheduled to work from 5 a.m. to 2:30 p.m. Flight 448 departed for Los Angeles at 2:39 p.m.
The plane had just departed when the pilots and first-class passengers heard pounding, the airline said. The captain immediately declared an emergency and turned back to Seattle-Tacoma International Airport. The plane was in the air just 14 minutes.
Medics checked the worker, an employee of Alaska Airlines contractor Menzies Aviation, and found he wasn't hurt, airport spokesman Perry Cooper said. The name of the worker has not been released.
The man was also evaluated at a hospital and passed a drug test after being released, airline spokesman Bryan Zidar said.
Officials for Menzies Aviation, an English company, did not immediately return messages from The Associated Press.
Airlines are increasingly turning over airport jobs to contractors to save money. Alaska Airlines outsourced bag-handling work at the Seattle airport in 2005.
The Service Employees International Union, which is trying to organize airport workers, said outsourcing has resulted in the hiring of low-wage workers who often hold more than one job and don't work for long as a baggage handler.
"These operations require a stable, well-trained workforce," said Heather Szerlag, a union official. "We see this as a significant problem."
Koenig reported from Dallas. Nicholas K. Geranios contributed to this report from Spokane, Washington.
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