DALLAS (AP) – Investigators say they found rivet holes that were too big and appear to be misshapen from wear in a Southwest Airlines jet that peeled open in flight earlier this month.

The National Transportation Safety Board said Monday that its examination of a section of the Boeing 737’s fuselage skin adjacent to where a 5-foot hole tore open revealed rivet holes that were too wide for the rivets. The rivets hold overlapping sections of the plane’s skin together.

The board also said in a statement that some rivet hole were worn into irregular shapes instead of being round.

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The board didn’t offer any conclusions based on the evidence. Its investigation is continuing.

The plane was at about 34,000 feet over Arizona with 118 people on board on April 1 when the hole opened. The pilot guided the plane to a safe emergency landing. There were no injuries.

Metal fatigue was initially suspected to have caused tiny subsurface cracks in the aluminum skin, which gave way during flight. Now investigators think the seeds of the near-disaster might have been planted when the plane was built.

After the Southwest incident, Boeing told airlines that owned about 190 other 737s built in the 1990s to immediately conduct electromagnetic inspections of an area of the roof called the lap joint, where overlapping panels of skin are riveted together.

Boeing Co. said Monday that inspections have been completed on about three-fourths of those planes, and only the five at Southwest were found to have cracks. Boeing said it was analyzing portions from panels of those planes “to validate the initial inspection findings,” but added no final conclusions have been drawn.

The Southwest plane had made about 39,000 flights. A senior Boeing engineer said this month that the company didn’t expect airlines would need to inspect the lap joints for metal fatigue until about 60,000 flights.

Southwest declined to comment. Flight 812 was the second Southwest jet to develop a hole in the roof in the past three years. The airline canceled nearly 700 flights this month after grounding 79 similar planes — one-seventh of its fleet — for inspections.

Last week, Southwest Airlines Co. CEO Gary Kelly said he saw no evidence that bookings had been hurt by the incident. Kelly, whose airline flies only 737s and is one of Boeing’s biggest customers, went out of his way to praise the aircraft maker’s speedy response, including designing repair jobs.

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