Neither the Federal Aviation Administration nor Boeing requires an intensive inspection to check for fatigue cracks on the section of passenger jets that tore open mid-flight Friday on a Boeing 737-300, ABC News has learned.
After the incident Friday, the revelation calls into question the inspection program for aging U.S. aircraft.
The 15-year-old Southwest Airlines jet was inspected in March 2010 with what is known as a D-check, the most comprehensive check for an airplane.
Based on modeling and previous flight experience of Boeing 737-300s, there was no feeling that cracks could develop in this area of the plane.
Those cracks, which can develop after repeated takeoffs and landings, may well have been what caused the fuselage to fail, according to some experts.
Southwest now says that what was seen with Flight 812 was a new and unknown issue.
"Boeing has since identified an inspection program for this section of the aircraft," Southwest said in a statement. "Based on this incident and the additional findings, we expect further action from Boeing and the FAA for operators of the 737-300 fleet worldwide."
The jet came apart at a seam, where two pieces of metal are riveted together, which is an especially dangerous location because the plane can essentially unzip, experts said.
"That is a very disturbing point because it's a place that can cause a great amount of damage to an airplane," former National Transportation Safety Board member and aviation maintenance expert John Goglia said.
After the incident Friday, Southwest Airlines ground 79 of its planes until they could under go further inspection.
The airline said today that small, subsurface cracks were found in three other airplanes, but as of 4 p.m. CT, 19 of the company's planes had undergone the intense inspection with no findings and were returned to service.
Tests of the remaining aircraft in the sub-fleet of 79 planes will continue for the next few days, Southwest said.
"This test is designed to detect any subsurface fatigue in the skin that is not visible to the eye," the airline said in the statement released earlier today.
As ABC News first reported, fatigue cracks were discovered when investigators got their first look at the break on the Southwest jet Saturday.
"We did find evidence of widespread cracking across this entire fracture surface," NTSB member Robert Sumwalt said today.
Sources told ABC News that they do not know why the cracks were missed during the D-check or couldn't simply be seen.
Sumwalt confirmed what ABC News first reported on "Good Morning America," that investigators examining the jet found "pre-existing fatigue along the entire fracture surface known as multisite damage" in the area that separated from the plane's roof.
At this point it is still unclear how those cracks got there, or whether they played any role in the ceiling of one of the airline's Boeing 737-300 jets tearing open in mid-air.
On Friday's harrowing flight, the roof opened up at 36,000 feet, oxygen masks dropped in front of passengers and at least one flight attendant passed out.
No one was seriously hurt in the incident and the plane, which was flying from Phoenix to Sacramento, Calif., made a rapid descent and an emergency landing at a military base in Yuma, Ariz.