While a National Transportation Safety Board investigation is still underway, NTSB officials confirmed that the uncontained engine failure aboard Southwest Airlines Flight 1380 was the result of a fan blade breaking from a crack near the fan's hub. The failure is similar to one that occurred on another Southwest flight in September 2016.

"The fan blade separated in two places," said NTSB Chairman Robert Sumwalt. "At the hub... there's a fatigue fracture where this #13 fan blade would come into that hub. It also fractured roughly halfway through it. But it appears the fatigue fracture was the initial event. We have the root part, but we don't have the outboard part. The crack was interior, so certainly not detectable from looking at it from the outside."

After that incident, the manufacturer of the engine—CFM International—issued a technical bulletin urging customers to conduct more frequent ultrasonic inspections of the fan in the type of turbofan engine used by Southwest's 737 Next Generation aircraft. In 2017, CFM even asked the FAA to enact a new rule requiring those checks. But Southwest Airlines opposed the proposed change to inspection frequency, stating in a comment to the FAA that it would take longer for the airline to comply because of the number of engines in its fleet:

SWA does NOT support the CFM comment on reducing compliance time to 12 months. SWA estimates there are 732 engines in the SWA population. Compliance time of 18 months will be needed to schedule and complete the required ultrasonic inspections. CFM's risk assessment... did not take credit for the number of fan blades already inspected in the fleet and the findings rate. SWA requests this risk assessment be updated to make a more data-informed AD mandated compliance time.

The FAA has not made a final determination on the rule; the comment period ended in October 2017. The European Aviation Safety Agency announced in March that it is considering a similar rule based on the report from the September 2016 Southwest flight incident.

The NTSB and other officials have also shared more details about the accident aboard Flight 1380. No debris—including acrylic from the shattered window in row 14—was found inside the aircraft, indicating that the window was shattered and blown out by the cabin pressure. Passengers reported hearing a loud pop. Jennifer Riordan, a 43-year-old executive at Wells Fargo in Albuquerque, New Mexico was sitting next to the window and was partially sucked out of it by the depressurization; she later died from blunt force trauma to the head, neck, and torso, according to an official from the Philadelphia Department of Public Health.

Meteorologists at the NTSB were able to track falling debris from the engine, and investigators have found parts at the predicted locations—about 65 miles northwest of Philadelphia, according to Sumwalt. Large parts of the cowling were located by people living in the area, as an NTSB spokesperson noted on the agency's Twitter account:

NTSB photo of a piece of the engine cowling from @SouthwestAir #flight1380. Thanks to the general public, these and other parts have been found. Anyone who has found additional pieces please contact witness@ntsb.gov pic.twitter.com/iV3gJDxf7m — NTSB_Newsroom (@NTSB_Newsroom) April 18, 2018

Sumwalt said that an inspection of the airplane and cowling parts showed that the engine debris had struck the wing. "We can see paint transfer—we can see blue paint transfer, we can see a little bit of red paint transfer, and sure enough, on the cowling, there is a red line that says 'hoist here' for maintenance to know where to hoist the engine... so we know that's some of that cowling coming off and hitting the leading edge of that wing."

Given the position of the window that was shattered, it's likely that part of the cowling glanced off the window after striking the wing, shattering it. But investigators have not yet determined the exact cause.