Federal investigators allege that a manufacturing lapse at a Boeing (BA) factory 15 years ago led to the mid-air fuselage rupture of a Southwest Airlines (LUV) jetliner earlier this month, according to The Wall Street Journal.

While officials told the Journal that it is too early to draw definitive conclusions, as further testing and data analysis could bring other issues to the forefront, the probe has become increasingly focused on an error deriving from the assembly-line.

The plane in question is a Boeing 737-300, which suffered a five-foot gash in the upper part of the cabin on April 1 at a cruising height of 34,000 feet. The plane’s cabin, with 122 on board, suffered a rapid decompression, though no one was seriously hurt, and the jet was able to make an emergency landing at a military base in Arizona.

Following the incident, Southwest grounded and started inspecting 79 of its oldest Boeing 737s. In addition to the plane that ruptured, five of the airline’s other older 737s were found to have fuselage cracks.

Investigators led by the National Transportation Safety Board are probing into the impact of riveting techniques and certain sealants used as far back as 1996, according to the Journal. They are also looking into factory tooling used to hold plane parts during assembly, the report said.

The reason for the push into manufacturing techniques is because the aircrafts with fuselage cracks were all assembled around the same time, according to the officials, though they noted that it could also be an issue of quality-control among Southwest.

The plane had reportedly logged about 39,000 takeoffs and landings, which is significantly less than Boeing experts estimated it could face serious metal fatigue, according to the Journal.

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