The Southwest Airlines jet forced to make an emergency landing at an Arizona military base Friday after a hole was torn in the roof of the passenger cabin suffered fatigue cracking, the National Transportation Safety Board announced Sunday.

 “There was pre-existing fatigue along the entire fracture surface,” NTSB member Robert Sumwalt said during a press conference. “We haven’t gone through to see exactly where the cracks are, but we did find evidence of widespread cracking along the entire surface.”

The announcement came after news that federal records show cracks were found and repaired a year ago in the frame of the Boeing 737-300.

The fracture area, reportedly 5 feet long by 1 foot wide, was located where the airplane’s skin overlaps together on the aircraft, at what’s called a lap joint. This area is held by three rows of rivets that run down the length of the aircraft.

“We have clear evidence that the skins separated at the lower rivet line,” Sumwalt said.

Investigators will examine the fractured surface with hand grinders and make a cut at the hole, an opening of 8 or 9 feet long and 2 feet wide. The cut out piece will then be shipped to Washington for detailed examination. Investigators say the cutting procedure will take a day to complete.

Data recordings of the incident show that the Boeing 737-300 began to descend 18 minutes after takeoff Friday, and flight crew immediately responded to the loss of cabin pressure by donning oxygen masks and declaring an emergency.

The aircraft descended from 36,000 feet to 11,000 feet in approximately 4 ½  minutes before it leveled off at 9,000 feet. The aircraft originally planned a landing in Phoenix, but diverted to Yuma, Arizona, after being briefed by cabin crew.

Friday’s incident forced Southwest Airlines to ground 79 planes and 300 flights Saturday. The airline is expecting similar cancellations on Sunday, according to spokesman Brandy King.

“We don’t know when flights will return back to normal,” said King.

No one was seriously injured Friday as the aircraft carrying 118 people rapidly lost cabin pressure and made a harrowing but controlled descent, landing safely near Yuma, Ariz., 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.

An Associated Press review of Federal Aviation Administration records of maintenance problems for the 15-year-old plane showed that in March 2010 at least eight instances were found of cracking in the aircraft frame, which is part of the fuselage. The records showed that those cracks were repaired.

It's not uncommon for fuselage cracks to be found during inspections of planes that age, especially during scheduled heavy maintenance checks in which they are taken apart so that inspectors can see into areas not normally visible.

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.

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, airline spokeswoman Linda Rutherford said. The planes that were grounded Saturday have not had their skin replaced, she said.

"Obviously we're dealing with a skin issue, and we believe that these 80 airplanes are covered by a set of (federal safety rules) that make them candidates to do this additional inspection that Boeing is devising for us," Rutherford said.

Julie O'Donnell, an aviation safety spokeswoman for Seattle-based Boeing Commercial Airplanes, confirmed "a hole in the fuselage and a depressurization event" in the latest incident but declined to speculate on what caused it.

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 is working closely with the NTSB, Southwest Airlines and Boeing to determine what actions may be necessary," the FAA said Saturday.

The 737-300 is the oldest plane in Southwest's fleet, and the Dallas-based company is retiring 300s as it takes deliveries of new models. But the process of replacing all the 300s could take years.

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. The plane made an emergency landing in Charleston, West Virginia. It was later determined that the hole was caused by metal fatigue.

In response to that incident, Southwest changed its maintenance plan to include additional inspections, which FAA reviewed and accepted, said John Goglia, a former NTSB member and an expert on airline maintenance. The details of the plan are considered proprietary and aren't made public, he said.

Four months before that emergency landing, Southwest had agreed to pay $7.5 million to settle charges that it operated planes that had missed required safety inspections for cracks in the fuselage. The airline inspected nearly 200 of its planes back then, found no cracks and put them back in the sky.

The Associated Press contributed to this report.