PHILADELPHIA (CBS) — Investigators with the National Transportation Safety Board are taking a closer look at the troubled engine of the Southwest Airlines jet that made an emergency landing in Philadelphia. One passenger aboard that plane was killed.

Several pieces of the plane have been found on the ground in Berks County, as a part of the cowling – the outer covering of the engine – has been recovered from a field in Bernville.

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The mid-air incident has broken a string of eight years without a fatal accident involving a U.S. airline.

Southwest Plane Debris Recovered In Berks County

The investigation is focusing on whether metal fatigue caused a fan blade in the plane’s engine to snap off, triggering a catastrophic chain of events. One blade on the engine’s front disc is missing.

“Shortly thereafter, the cabin altitude warning horn activated, and from my experience of flying a 737, I can tell you that warning activates around 13,500, 14,000 feet, somewhere in there,” said NTSB chairman Robert Sumwalt.

From when the engine blew until the plane landed took 22 minutes. Passenger Jennifer Riordan was partially sucked out of a blown-out window and died from her injuries.

The NTSB has been looking at the inspection techniques currently in place and checking to see if it needs to be changed.

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The damaged Southwest Airlines plane sits inside a hangar at the Philadelphia International Airport, but 70 miles north in an open field in Bernville Borough, Berks County, a Gaming Commission employee found a piece of plane wreckage belonging to Flight 1380. Crews picked it up and waited for the FBI to arrive.

Air traffic control officials located the debris by watching it detach from the plane on a radar.

“They’re fairly light parts so they’re going to be more aerodynamic and float more. We have not found any internal engine components at this time,” said Sumwalt.

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Sumwalt says the investigation will take 12 to 15 months.

“We will do a very methodical investigation. Our purpose for being here on scene is to document the perishable evidence, the information that can go away with the passage of time,” said Sumwalt.

The NTSB has been investigating since Riordan, a married mother of two and Wells Fargo bank executive from New Mexico, died when the Boeing 737 jet engine failed midflight and blew out the window.

“On behalf of the entire Southwest family, I want to extend my deepest sympathies for the family of the deceased customer,” said Southwest Airlines CEO Gary Kelly.

Kelly told the NTSB that the airline will begin enhanced inspections of its fleet very soon.

Flight 1380 was powered by CFM 56 engines – a common engine found on 737s. The engine company has sent staff to the scene to support the investigation.

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The NTSB says a preliminary examination of the blown jet engine revealed that the engine’s fan blades were separated and missing, which means there was metal fatigue.

The plane flew some 70 miles with a single working engine.

Southwest last checked the plane on Sunday.

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The NTSB will have a meeting with Southwest officials to go over all of their maintenance records.