Investigators are breaking out their magnifying glasses and microscopes to further scrutinize the Southwest Airlines plane that made an emergency landing earlier this week after a gaping hole in the plane's ceiling opened in midair.
The section of the plane from which the hole emerged was cut out of the aircraft for further inspection and arrives today in Washington.
Investigators at the National Transportation Safety Board labs will examine the metal under a microscope to see if there is any evidence of minuscule cracks that could have caused the problem.
ABC News has learned that so far investigators can't find any obvious signs of corrosion or metal weakness in the area of the 14-by-17-inch hole.
They'll also assess the plane's black boxes -- the cockpit voice recorders and the flight data recorder -- as they begin a detailed analysis of what went wrong.
The Southwest 737-300 is 15 years old and had more than 42,500 takeoffs and landings. It appears the plane was not yet part of a federal program that subjects older planes to intense inspections for cracks and weaknesses.
Southwest Inspections This Week Found No Other Problem Planes
According to Southwest, the plane forced to land Monday at Charleston's Yeager Airport in West Virginia was last inspected as part of routine maintenance in January 2009.
Monday night, Southwest visually inspected all 180 of its 737-300 series jets.
"Inspections of all our 737-300's were completed last night with zero findings," the airline announced Tuesday.
"We're working as quickly as possible to figure out what happened to this plane," Federal Aviation Administration administrator Randy Babbitt said in a Tuesday statement. "We'll be looking closely at all the FAA safety directives that applied to this aircraft, as well as the plane's maintenance history."
But maintenance expert and former NTSB member John Goglia criticized the airline and government regulators alike.
"Where's the maintenance programs with this airline? Where's the FAA? Where's the oversight?" Goglia said Tuesday. "I mean, the list goes on and on. These people on this airplane really don't know how lucky they were."
Southwest was quick to defend its safety record.
"Southwest Airlines has an exemplary safety record that always is our focus," Southwest spokeswoman Whitney Eichinger responded in a Tuesday statement. "We are actively engaged with the NTSB in finding the cause of this incident and assuring that it does not happen again. We applaud our pilots and flight attendants for their expert handling of this situation and our customers for their cooperation."
Three other domestic airlines also fly 737-300s: Continental, United and USAirways. All have two dozen or fewer of the planes in their fleets. So far, only Continental has said it is giving those jets a closer look.
Southwest Passengers Recount Emergency Landing
The hole in Southwest flight 2294 opened with a "pop" about 30 minutes into Monday's flight en route from Nashville, Tenn., to Baltimore.
"The pressure drop was obvious and pretty immediate, and the oxygen masks dropped," passenger Steve Hall of Murfreesboro, Tenn., told ABCNews.com Tuesday, adding that the hole was two rows away from his seat. "Everybody was a little squirmy of course."
"We could see the ceiling tile that had basically been sucked into the fuselage area, and we saw some wires and things that would run from the front to back of the plane and then it got very foggy, similar to what you'd see with dry ice," passenger Erin O'Donovan said. "And then, at the same time, the plane was lowering, so you could definitely feel us going down to a lower altitude level."
Hall and O'Donovan were among the 131 people on board when the plane landed safely. No one was injured during the scare.
In 1988, an Aloha Airlines 737 peeled apart at 24,000 feet. One flight attendant died, and another eight people were injured in the incident. That was when the FAA realized that older planes could suffer metal fatigue and needed frequent inspections.
Just last year, Southwest was slapped with a hefty $10.2 million fine, later reduced to $7.5 million, when it was discovered that the airline had failed to inspect 46 of its Boeing 737s as required.
The mandatory inspections were designed to uncover any cracks in the body of a plane. When the airline finally did inspect the planes, it found cracks in six of them.