In an official statement, the airline confirmed that the L.A.-bound flight returned to Seattle when the pilot "reported hearing banging from beneath the aircraft" immediately after takeoff. According to KIRO-TV reporter Alison Grande, passengers also heard screaming.
Upon landing, an airport employee walked out of the plane's pressurized and temperature-controlled front cargo hold, reportedly telling authorities "he had fallen asleep." The man was then taken to a local hospital as a precaution and released shortly afterward.
Alaska Airlines says they are "actively investigating" the incident.