U.S. investigators suspect a manufacturing lapse at a Boeing Co plant 15 years ago could be to blame for a midair tear in a Southwest Airlines Co aircraft earlier this month, the Wall Street Journal reported on its website on Sunday.

On April 1, the Boeing 737 was heading from Phoenix to Sacramento, California, when a 5-foot tear opened up 20 minutes after takeoff along the roof just above the left wing. The incident led U.S. regulators ordering airlines to inspect older model Boeing 737 aircraft for cracks.

Southwest found fuselage cracks in five other older Boeing 737-300 aircraft in its fleet.

The paper reported the fuselages are built at a Kansas factory that Boeing owned in 1996.

Investigators led by the National Transportation Safety Board are looking into the potential impact of riveting techniques and certain sealants going back to around that time, the Journal said, citing government and industry officials.

They are also looking at tools used to hold aircraft parts during assembly, the paper said, adding that officials said it was too early to draw final conclusions.

In an e-mailed statement, Boeing said it continues to work closely with the regulators as the investigation continues into the root cause of the incident.

The company said so far inspections have been completed worldwide on about 75 percent of the 190 airplanes affected.

No conclusions have been reached about the root cause of the inspection findings, nor of any relationship to the April 1 event, it said. Any attempt to draw conclusions on either would be premature and speculative.

(Reporting by Paritosh Bansal; Editing by Dhara Ranasinghe)