Investigators believe the problems may have started 15 years ago on an assembly line, where rivets were created incorrectly.
ABC News has learned investigators are focusing their attention on rivets -- those tiny pins that hold pieces of a plane together. Sources say on the Southwest 737 that ripped open the rivets holding parts of the plane's fuselage together, failed because they didn't fit perfectly. The rivet holes themselves may have been sized incorrectly back on the assembly line.
"They would over time with the flexing and expansion and contraction of pressurizing the plane's fuselage when it takes flight, those holes will microscopically expand or change," said ABC7 aviation consultant Ron Wilson.
And in this plane's case the seam tore creating a gapping five-foot hole. Oxygen masks dropped and the pilot quickly dove to a lower altitude.
As a result, Boeing ordered inspections of similar 737s worldwide -- nearly 600 total. A third required inspections immediately and of those, five have slight cracks. All are Southwest jets and sources say all five were built around the same time as the plane that came apart.
"The cracks are along this joint here where the two pieces lap," said Wilson. "That crack will become worse and worse with every pressurization."
Passengers ABC7 spoke with have mixed reactions about the latest's findings.
"We take chances when we fly anyway and there can be problems that occur occasionally, but I still believe in the system," said Kathy Belville from San Diego.
"They should have found it earlier. I mean, they're not strict enough on their regulations," said Kamilla Sadkykhova from Daly City.
"I don't see there being an issue at all," said Brent Kinser form Phoenix.
"They should be taking them out of service until they investigative them," said Johnny Choumas from San Francisco.
The investigation is still going on. In fact the plane that ripped has been patched and is expected to go back in service.