The tear along a riveted "lap joint" shows evidence of extensive cracking that hadn't been discovered during routine maintenance before Friday's harrowing flight -- and probably wouldn't have been unless mechanics had specifically looked for it, officials said.
National Transportation Safety Board investigators were overseeing the removal of the top section of the jetliner's roof around the 5-foot long tear and will send the structure to Washington, D.C., for analysis.
Meanwhile, Southwest said it had cancelled about 300 flights for the second day in a row Sunday as it inspected 79 similar planes in its fleet that it has grounded. The NTSB said it had not been notified of similar problems cropping up during those inspections. Southwest has not said if it has found other problems.
No one was seriously injured Friday as the aircraft carrying 118 people rapidly lost cabin pressure and made a controlled descent from 34,400 feet, landing safely at the airport in Yuma, 150 miles southwest of Phoenix.
But passengers recalled tense minutes after a hole ruptured overhead with a blast and they fumbled frantically for oxygen masks as the plane descended.
NTSB board member Robert Sumwalt said Sunday that the rip was a foot wide, and that it started along a joint where two sections of the 737's skin are riveted together. An examination showed extensive pre-existing damage along the entire tear.
But Sumwalt noted that the extensive cracking, known in the industry as "multi-site damage," could not have been spotted during routine maintenance.
The NTSB could issue urgent recommendations for inspections on other 737s if investigators decide there is a problem that has been overlooked. The type of riveted joint involved is not normally subjected to extensive checks for wear or fatigue.
Federal records show cracks were found and repaired a year ago in the frame of the same Southwest plane.
An Associated Press review of Federal Aviation Administration records of maintenance problems for the 15-year-old plane showed that a March 2010 inspection found 10 instances of cracking in the aircraft frame, which is part of the fuselage, and another 11 instances of cracked stringer clips, which help hold the plane's skin on.
The records show the cracking was either repaired or the damaged parts replaced. Cracking accounted for a majority of the 28 problem reports filed as a result of that inspection.
It's not uncommon for fuselage cracks to be found during inspections of planes that age, especially during scheduled heavy maintenance checks in which planes are taken apart so that inspectors can see into areas not normally visible.
The jetliner had gone through about 39,000 cycles of pressurizing, generally done for takeoffs and landings. Cracks can develop from the constant cycle of pressurizing for flight, then releasing the pressure.
Southwest officials said the Arizona plane had undergone all inspections required by the FAA. They said the plane was given a routine inspection Tuesday and underwent its last so-called heavy check, a more costly and extensive overhaul, in March 2010.
The plane remains at Yuma and the NTSB and Boeing will oversee the work.
The decompression happened about 18 1/2 minutes after takeoff from Phoenix Sky Harbor International Airport after the pilots reached their cruising altitude. They immediately donned their oxygen masks, declared an emergency and briefly considered returning to Phoenix before the cabin crew told them of the extent of the damage, Sumwalt said.
"They discussed landing in Phoenix but quickly upon getting the assessment decided to divert to Yuma because it was the closest suitable airport," he said.
The plane's voice and data recorders were being examined in Washington, and Sumwalt said they worked well but show no sign of a problem before the incident.
Southwest operates about 170 of the 737-300s in its fleet of about 540 planes, but it replaced the aluminum skin on many of the 300s in recent years, a Southwest spokeswoman said. The planes that were grounded Saturday have not had their skin replaced.
A total of 288 Boeing 737-300s currently operate in the U.S. fleet, and 931 operate worldwide, according to the Federal Aviation Administration. The FAA declined to say Sunday if it was requiring other operators to check their aircraft for similar flaws.
A similar incident happened in July 2009 when a football-sized hole opened up in-flight in the fuselage of another of Southwest's Boeing 737s, depressurizing the cabin. Sumwalt said Sunday that the two incidents appear to be unrelated.
Friday afternoon, Flight 812 was only a few minutes into its trip from Phoenix to Sacramento, Calif., and flight attendants were taking drink orders when the explosion rocked the cabin.
Shawna Malvini Redden covered her ears, then felt a brisk wind rush by. Oxygen masks fell, the cabin lost pressure, and Redden, now suddenly lightheaded, struggled to maneuver the mask in place.
Then she prayed. And, instinctively, reached out to the stranger seated next to her in Row 8 as the pilot of the damaged aircraft began a rapid descent from about 34,400 feet in the sky.
"I don't know this dude, but I was like, 'I'm going to just hold your hand,"' recalled Redden, a 28-year-old doctoral student at Arizona State University.