Evidence of widespread fuselage cracks and fatigue have been found on a Southwest Airlines passenger jet that made an emergency landing in Arizona with a hole in the cabin.
The incident on Friday prompted Southwest, which carries more people than any other American domestic airline, to ground planes over the weekend so it could inspect its older model 737-300s, Reuters reports.
The airline says small sub-surface fuselage cracks have been found in three other planes.
A United States safety investigator says they are trying to establish how well the aircraft involved in Friday's incident was maintained.
Southwest paid a $US7.5 million Federal Aviation Administration fine for operating 737s without required fuselage structural inspections in 2006-07.
The 737-300 represents roughly 20% of Southwest's all-737 fleet, the most popular commercial aircraft ever and a workhorse globally.