Now the question is: Are other Boeing 737s at risk?
The Southwest flight was at 34,000 feet when a 5-foot hole ripped open and the oxygen masks dropped.
"It was frightening, very frightening," said one Southwest flight passenger after that April 1 flight.
Initially, investigators thought it was metal fatigue after the plane's 39,000 take-offs and landings. On Friday, however, ABC News learned that the problem may have been a defect when the Boeing 737 was manufactured in 1996.
Chicago-based Boeing's response reads, in part: "No conclusions have been reached about [sic] the root cause of the inspection findings, nor of any relationship to the April 1 event. Any attempt to draw conclusions on either would be premature and speculative?"
When asked what this means for Boeing, a transportation expert with DePaul University Professor Joe Schwieterman said: "All eyes point to Boeing. This was a defect upon delivery. It's embarrassing for Boeing. It's somewhat unprecedented."
Specifically, investigators are focused on the rivets, or the metal pins that hold the plane together.
Sources tell ABC that at a seam in the fuselage, where one metal piece overlapped another, the rivet holes were not sized correctly. They were not fastened together tight enough, which over time stressed that area and cracked the plane.
After that incident, Boeing ordered inspections of 190 planes. Boeing says 75 percent have been looked at. Only five -- all Southwest planes -- had slight cracks.
Southwest operates 548 Boeing 737s, and has the most daily departures out of Midway Airport.
"What are you going to do, drive back and forth twice a week from Chicago to Houston? No. You got to take your chances," said Southwest passenger Michael Gutierrez.
Boeing says it continues to work closely with federal investigators. The plane that ripped open has been repaired, and is expected to go back into service.