At around 34,000 feet, a rip five-feet long, and one-foot wide, opened up in the fuselage. The National Transportation Safety Board said on Sunday the rupture started along a joint where two sections of the skin are riveted together. Investigators are cutting out the fractured area so it can be examined more closely. So far, they see no sign of faulty maintenance.
ABC has learned the FAA doesn't require airlines to do intensive searches for such cracks, because it didn't believe they could happen.
"This is basically something that comes about with age and with utilization, and that's what they'll be focused in on. There's absolutely no indication here that this is something that's fleet-wide," said ABC News Aviation Consultant John Nance.
Even so, Southwest is inspecting dozens of Boeing 737 airplanes and canceled another 300 flights on Sunday, nationwide. At the San Francisco International Airport, there were plenty of unhappy passengers.
Six-hundred flights in all were cancelled over the weekend. There were some passengers who have been trying to get home for two days now. The best advice is if you have a Southwest flight to catch, call ahead before you come to the airport.
The unlucky half of the high school jazz band trying to get home to Omaha, Nebraska were holding tickets for a cancelled Southwest flight and are now stranded in San Francisco. Part of the band was lucky, since they booked their tickets on a different airline and are headed home.
"We already looked at trying to charter a bus. It would be $9,000. We are a public high school, it's not like we can be writing checks," said Kristine Gerber, a jazz band chaperone.
Other passengers on cancelled flights had no choice but to pay big bucks to buy last minute tickets on different airlines. Roxanne Kincade from Kansas City couldn't wait another day, so she went to the Delta counter.
"They were doubling and tripling the price. So we spent $1,000 to get home," said Kincade.
As federal inspectors investigate Friday's emergency landing of a Southwest flight in Arizona, the airline cancelled 300 flights for a second day in a row to inspect 79 of its Boeing 737s -- planes that have not had their aluminum skin replaced. The inspections are expected to take several days.
"Was the aircraft well maintained and should it have been maintained better? That is exactly why we are here to look at why this problem occurred, why it was not detected, and why this event happened," said NTSB board member Robert Sumwalt.
In Oakland, there was plenty of confusion and the long waits to go with it. Whether the destination was Phoenix, Baltimore, Las Vegas, or Santa Ana, the boards all said the same thing -- cancelled. And passengers said pretty much the same thing too... they were very frustrated and didn't know what they were going to do.
So far in the Bay Area, Oakland was hit the hardest with 30 flights cancelled there. There have been seven outbound flights out of SFO cancelled Sunday. The inspection of the 737s is expected to last until Tuesday, but there's no word yet from Southwest about Monday's flights.