SEATAC, Wash. -- An Alaska Airlines MD-83 bound for Burbank, Calif., returned to Seattle-Tacoma International Airport Monday after a hole in the plane's fuselage caused the jet's cabin to lose pressure, KIRO 7 Eyewitness News reported.

It wasn't immediately clear what caused the 12- by 6-inch hole near the cargo door on the plane's left side, but a union official told KIRO 7 Eyewitness News that a non-union baggage handler operating a conveyor machine knocked the hole in the jet.

Flight 536 left Sea-Tac at 3:54 p.m. Passengers heard a loud bang as the aircraft was climbing to cruising altitude, a Federal Aviation Administration representative said. The aircraft lost pressurization about 20 minutes into the flight and oxygen masks deployed. The jet landed at Sea-Tac at 4:53 p.m.

None of the 140 passengers or crew members on board were hurt. Passengers were put on a different plane and taken to Burbank.

Both the FAA and the National Transportation Safety Board are investigating, KIRO 7 Eyewitness News reported.

The International Association of Machinists and Aerospace Workers, which represents baggage handlers, has been in a dispute with Alaska Airlines over the outsourcing of the company's baggage handling operations. Nearly 500 baggage handlers were laid off in May in a move the company said would save $13 million a year.