An airport worker who fell asleep and found himself trapped in a plane's cargo hold forced a Los Angeles-bound Alaska Airlines flight to return to Seattle on Monday afternoon.

Flight 448 had just taken off when the pilot and some first-class passengers heard screaming and banging from down below, the airline said. The captain immediately returned to Seattle-Tacoma International and declared an emergency for priority landing.

After the plane landed a ramp agent came out from the front cargo hold, which Alaska said is pressurized and temperature-controlled.

"Upon exiting, he told authorities he had fallen asleep," the airline said.

The man, an employee of Alaska contractor Menzies Aviation, walked off the plane. He was initially checked by medics at the airport and found to be unhurt, airport spokesman Perry Cooper said.

The man was also checked at a hospital and released, the airline said Monday evening. After his release from the hospital, he passed a drug test, airline spokesman Bryan Zidar said.

The airline did not identify the worker. A Menzies spokesman did not immediately return an email request for comment and details about the employee.

The plane was airborne for 14 minutes.

The man started work at 5 a.m. Monday and was due off at 2:30 p.m., the airline said in a statement. He had been part of a four-person team loading baggage onto the flight, which initially departed at 2:39 p.m.

Before the plane departed, the leader of the man's team noticed he was missing, called into the cargo hold for him and called and texted the man's cellphone but got no answer, the airline said. Co-workers decided he had finished his shift and gone home.

"The passengers in first class heard banging from underneath us and a person yelling for help," passenger Jesse Sycuro told KING-TV of Seattle.

A short time later, passengers heard the announcement that the plane was going to turn around.

Zidar said Monday night he understood that both first-class passengers and the pilots heard the banging.

The Federal Aviation Administration is investigating, spokesman Allen Kenitzer confirmed in an email.

All ramp employees undergo full criminal background checks and drug checks before hiring and are subject to random drug tests during their employment, the airline statement said.

The plane carried 170 passengers and six crew members.

The flight departed again at 3:52 p.m. and arrived at a terminal gate in Los Angeles at 6:29 p.m., more than an hour late, the airline said.