From the WallStreet Journal:
Investigators suspect that a manufacturing lapse at a Boeing Co. factory 15 years ago is why the fuselage of a Southwest Airlines Co. jetliner ruptured in midair this month, according to government and industry officials. It is too early to draw definitive conclusions, the officials said, and further testing and data analysis could bring other issues to the forefront. But the federal probe increasingly is focused on some type of assembly-line lapse—a rare occurrence in modern aircraft production— that would explain an incident that stunned the airline industry and worried travelers.
To read the full story in the WSJ click the link.