"It's very unusual, " said National Transportation Safety Board chairman Robert Sumwalt who is supervising the investigation in Philadelphia. "We are taking this event extremely seriously - this should not happen."
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One passenger was nearly sucked out of the Boeing 737 on Tuesday morning when the left engine failed in midair. She was seated near the engine and killed after being hit by shrapnel from the exploding parts.
A preliminary examination of the GE engine showed evidence of metal fatigue, according to NTSB officials who said a fan blade broke off from its hub during flight. Investigators looking at the broken edge of the blade say they found crack lines consistent with metal fatigue.
"The plane start to take a dive and then a few minutes later the window became open and then there was commotion from the passengers at that point because we didn't know what was going on," said passenger Cherub Ruth, one of 149 people on board the plane. "It was one passenger but there was so many things happening at the same time we weren't sure. And from there on it was scary."
Because Southwest planes travel multiple short flights each day, with quick turnarounds, there is additional stress on their engines, engineers said. Tuesday's accident occurred en route from New York to Dallas. The plane was scheduled to travel onto San Francisco, where it was to depart on Wednesday and land at Chicago Midway about 11 a.m.
WATCH: Southwest plane in fatal engine blast had Chicago on schedule
Southwest plane in fatal engine blast had Chicago on schedule
What caused the metal fatigue on that aircraft is the core question for federal investigators.
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Chicago aircraft materials expert Sammy Tin at Illinois Institute of Technology said one answer may be in the Southwest business model.
"Southwest Airlines planes, they operate on a much more intense flight cycle. A lot of their flights are very short and due to the fact that these flights are short, they have to cycle the engine on and off many, many times," Tin told the ABC7 I-Team. "So that accelerates the amount of fatigue damage that can accumulate in these types of materials."
Southwest had an uncontained engine failure in another 737 two years ago, and some aviation experts are concerned that a second, similar accident may warrant an industry-wide examination.
When jet engines fail, the design is intended to contain metal debris and prevent rocketing pieces.
Even though that didn't work in Tuesday's explosion at 30,000 feet, Southwest's CEO Gary Kelly is urging restraint.
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"We don't know the cause of this incident and it's premature to even link it to other engine failures that have occurred," Kelly said.
Southwest executives said that it is accelerating inspections of the engine type that was involved in the accident. Those inspections are likely to be completed within 30 days. The tragedy was the airline's first on-board passenger fatality due to an accident in its history.
A piece of the engine from SW flight 1380 was found 60 miles away from Philadelphia's airport, where the jetliner made an emergency landing.
FBI agents and other law enforcement officers continue to search Berks County for debris from the plane that could help explain what happened. "In aviation, there should be inspection techniques and procedures in place to detect something like that. What we want to find out is why was this not detected ahead of time," the NTSB's Sumwalt said on Wednesday.
The passenger killed was identified as Jennifer Riordan, 43. She worked for Wells Fargo in Albuquerque, New Mexico, according to the bank, and is survived by her husband and two children.
Late Wednesday afternoon, officials said Riordan died of "blunt impact trauma of the head, neck and torso." Her manner of death was listed as an accident.