The aircraft had been operating flight 2294 from Nashville to Baltimore-Washington but diverted to Yeager Airport near Charleston, West Virginia.
Southwest says it has sent maintenance personnel to Yeager to assess the aircraft and assist the National Transportation Safety Board in determining the cause of the depressurisation.
© Steve Hall
Initial crew reports, says the carrier, indicate that the incident is related to a "small-sized hole located approximately mid-cabin", near the top of the aircraft.
Images carried by local Charleston media indicate that the hole is in the upper fuselage just ahead of the vertical fin. An internal photo purporting to show the damage also indicates it to be around seat row 21, which would be situated about three rows from the back of the cabin.
© Steve Hall
Southwest has not confirmed the serial number of the aircraft involved, although an unconfirmed report indicates it is 26602, a 15-year old airframe.
None of the 126 passengers and five crew members was injured in the event, which occurred around 30min into the 16:05EST service and prompted deployment of oxygen masks.
But Southwest Airlines says it has initiated an inspection of all 737-300 aircraft, in an "abundance of caution", adding that it expects minor disruption to its schedule today while this is completed.
Southwest's oversight regime ran into problems last year when the US FAA stated that it had been operating nearly 50 737 aircraft without meeting requirements for repetitive inspection to detect fatigue cracks in the fuselage.
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