Yuma, Ariz. ? Inspectors have found small, subsurface cracks in three more Southwest Airlines planes that are similar to those suspected of causing a jetliner to lose pressure and make a harrowing emergency landing in Arizona, a federal investigator said Sunday.

Southwest said in statement that two of its Boeing 737-300s had cracks and will be evaluated and repaired before they are returned to service. A National Transportation Safety Board member told The Associated Press later Sunday that a third plane had been found with cracks developing.

The cracks found in the three planes developed in two lines of riveted joints that run the length of the aircraft.

Nineteen other Boeing 737-300 planes inspected using a special test developed by the manufacturer showed no problems and will be returned to service. Checks on nearly 60 other jets are expected to be completed by late Tuesday, the airline said.

That means flight cancelations will likely continue until the planes are back in the air. About 600 flights in all were canceled over the weekend.

NTSB board member Robert Sumwalt said Boeing was developing a “service bulletin” for all 737-300 models with comparable flight cycle time as the Arizona jet, which was 15 years old and had about 39,000 takeoff and landing cycles.

There are 931 such models in service worldwide, 288 of which are in the U.S. fleet.

Boeing’s bulletin would strongly suggest extensive checks of two lines of “lap joints” that run the length of the fuselage. The NTSB has not mandated the checks, but Sumwalt said the FAA will likely make them mandatory.

Friday’s flight carrying 118 people rapidly lost cabin pressure after the plane’s fuselage ruptured — causing a 5-foot-long tear — just after takeoff from Phoenix.

Passengers recalled tense minutes after the hole ruptured overhead with a blast and they fumbled frantically for oxygen masks. Pilots made a controlled descent from 34,400 feet into a southwestern Arizona military base. No one was seriously injured.

The tear along a riveted “lap joint” near the roof of the Boeing 737 above the midsection shows evidence of extensive cracking that hadn’t been discovered during routine maintenance before the flight — and probably wouldn’t have been unless mechanics specifically looked for it — officials said.

“What we saw with Flight 812 was a new and unknown issue,” Mike Van de Ven, Southwest executive vice president and chief operating officer, said. “Prior to the event regarding Flight 812, we were in compliance with the FAA-mandated and Boeing-recommended structural inspection requirements for that aircraft.”

Sumwalt said that the rip was a foot wide, and that it started along a joint where two sections of the plane’s skin are riveted together. An examination showed extensive pre-existing damage along the entire tear.

The riveted joints that run the length of the plane were previously not believed to be a fatigue problem and not normally subjected to extensive checks, Sumwalt said.

“Up to this point only visual inspections were required for 737s of this type because testing and analysis did not indicate that more extensive testing was necessary,” Sumwalt said.

That will likely change after Friday’s incident, he said.

The FAA declined to say if it was requiring other operators to check their aircraft for similar flaws.