Investigators found a major clue to what may have caused Southwest jet's engine failure

Amanda Bourman/AP

The engine on a Southwest Airlines plane is inspected as it sits on the runway at the Philadelphia International Airport after it made an emergency landing in Philadelphia, Tuesday, April 17, 2018.

• NTSB chairman Robert Sumwalt said the failed engine on the Southwest Airlines jet is missing a fan blade.
• Investigators found evidence of metal fatigue at the point where the blade broke off.
• Southwest Airlines will immediately begin inspections of its entire fleet of more than 710 Boeing 737s.

Investigators from the National Transportation Safety Board have a major new clue in their hunt for what caused an engine on a Southwest Airlines Boeing 737 to fail mid-flight. The incident resulted in the death of 43-year-old Jennifer Riordan of Albuquerque, New Mexico.

In a press conference Tuesday evening, NTSB chairman Robert Sumwalt announced that one of the engine's 24 fan blades had snapped off near the central hub. To be specific, it's the 13th of 24 blades. 

According to Sumwalt, a preliminary examination of where the blade failed found evidence of metal fatigue. The entire investigation is expected to take 12-15 months. 

However, Sumwalt told the media Southwest Airlines CEO Gary Kelly indicated the airline will immediately begin enhanced inspections of the engines across its entire fleet more than 710 Boeing 737s using ultrasonics. 

The Boeing 737-700 involved in the incident was delivered new to Southwest Airlines in July 2000. The 17.8-year-old jet is powered by a pair of CFM International CFM56-7B turbofan engines. CFM International is a joint venture between GE Aviation and France's Safran Aircraft Engines. 

The CFM56-7B is one of the most popular jet engines in the world and can be found on 6,700 aircraft around the world. 

"The members of the CFM Team worldwide wish to express their deepest condolences to the family of the victim of this incident," the jet engine maker said in a statement. "CFM will support the NTSB and Southwest Airlines in determining the cause of the accident and CFM and its parent companies, GE and Safran, will make every resource necessary available to ensure support."

Read More about the Southwest Incident:

• Southwest passenger's death was the first fatality on a US passenger airline in more than 9 years
• Southwest pilot to air traffic control before emergency landing: 'There's a hole and someone went out'
• Southwest passenger says there was 'blood everywhere' after 'terrifying' emergency landing
• Southwest passenger who died after major engine failure has been identified as a Wells Fargo VP and mother of two