After the April 1 incident, investigators found widespread cracking in the metal fuselage. The initial thought was that the cracks were caused by metal fatigue, a result of the plane's 39,000 takeoffs and landings.
But now it appears it could be a problem in production, sources said. The plane was manufactured in 1996 and investigators are focused on rivets, thousands of metal pins that hold the pieces of an airplane together.
Investigators believe that in the area of the plane's skin that failed, those pieces were not held together as they should have been.
Some of the rivet holes were not sized correctly and the two pieces that separated were not fastened together tightly enough at the seam, sources said. Over time, it's believed, the area was repeatedly stressed, resulting in the cracking.
Even as the investigation continues, the damaged plane has been patched and it's expected to go back into service.
After the incident, Boeing ordered inspections of the nearly 600 similar 737s worldwide, but only 190 were required to have inspections right away.
About 75 percent of those planes have been looked at and only five planes were found to have slight cracks, Boeing told ABC News tonight.
"Portions of the panels from those airplanes have been shipped to Boeing, and we are conducting analyses to validate the initial inspection findings," said a statement by Doug Alder, a Boeing spokesman.
Most of those planes were built about the same time as the aircraft whose fuselage came open, sources said.
But, Boeing emphasized, the investigation continues in conjunction with the NTSB.
"No conclusions have been reached about root cause of the inspection findings, nor of any relationship to the April 1 event," Alder said. "Any attempt to draw conclusions on either would be premature and speculative."