Investigators trying to determine why the roof of a Southwest Airlines jet cracked open in flight have issued preliminary findings suggesting there may have been flaws in the riveting work when Boeing built the plane 15 years ago.

The National Transportation Safety Board said some of the rivets used to bind the Boeing 737’s aluminium panels together were sunk in holes larger than the rivet shafts. The holes were not lined up correctly and were misshapen, not round, the board said.

It did not offer any conclusions and said the investigation was continuing.

Southwest Flight 812 was about 34,000 feet over Arizona with 117 passengers on board on April 1 when a 5ft-long hole opened in zipper-like fashion along a row of rivets in the roof. The pilot guided the plane to a safe emergency landing and there were no injuries.