YUMA, Ariz. (AP) - Southwest Airlines mechanics were working Sunday to cut out a section of ruptured fuselage from a Boeing 737-300 that was forced to make an emergency landing at a southwestern Arizona military base.

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 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.