"All of a sudden we heard all this pounding underneath the plane and we thought there was something wrong with landing gear," said Robert Higgins, from Jacksonville, Oregon.
The banging came from a ramp agent who told authorities he had fallen asleep in the front cargo hold before the flight took off, according to a statement from Alaska Airlines.
"At that point, we started hearing yelling, screams for help, very, very faint. That's when we notified the flight attendant that there was somebody underneath us," Seattle resident Jamie Davis said.
Flight 448, which departed at 2:39 p.m., immediately flew back to Seattle. It was in the air for 14 minutes, airline officials said.
Upon landing, authorities found a ramp agent inside the front cargo hold. The cargo hold is pressurized and temperature-controlled.
The ramp agent was not injured, but was taken to a hospital as a precaution and later released. He passed a drug test Monday afternoon. No charges were expected to be filed.
The employee was part of a four-person team loading the baggage for the flight. He had started work at 5 a.m. and was scheduled to finish his shift at 2:30 p.m.
Before departure, his team lead noticed the ramp agent was missing and tried to call his cell phone, but could not reach him. They thought he finished his shift and left.
The incident is under investigation. The flight ultimately landed in Los Angeles at 6:17 p.m.