NEW YORK (Reuters) - Federal investigators on Thursday said the engine fan blade that broke apart during a Southwest Airlines flight last month showed signs consistent with metal fatigue, as inspectors probe the first passenger fatality on a U.S. airline in nearly a decade.

FILE PHOTO: A U.S. NTSB investigator is on scene examining damage to the engine of the Southwest Airlines plane in this image released from Philadelphia, Pennsylvania, U.S., April 17, 2018. NTSB/Handout via REUTERS/File Photo

The accident touched off a scramble to inspect similar equipment. U.S. regulators this week broadened a directive to check the fan blades in hundreds of engines similar to the one that blew apart.

The identification of metal fatigue, the degradation of a metal’s structural integrity over repeated use, could necessitate more frequent and more comprehensive inspections of these parts.

The National Transportation Safety Board (NTSB) said it found six crack lines on the fan blade that separated and tore through the engine’s cowling during Southwest Flight 1380.

The NTSB’s investigation is ongoing to determine how long it takes for fan blades begin to show signs of fatigue and whether current inspection measures can effectively detect cracks.

“We continue to cooperate fully with the National Transportation Safety Board’s ongoing investigation into flight 1380. Safety is our top priority at Southwest, and we appreciate the efforts of the NTSB Investigators in developing today’s Preliminary Report,” Southwest said in a statement.

Southwest has been under intense scrutiny since April 17, when a fan blade on the Boeing 737’s CFM56 jet engine broke apart mid-flight, shattering a window and nearly sucking a woman out of the plane. The woman died of her injuries, marking the first U.S. airline passenger fatality since 2009.

The Federal Aviation Administration on Tuesday ordered additional inspections of fan blades in hundreds of additional engines similar to the one that failed in the Southwest incident. [nL1N1S80FH]

Southwest has said it planned to complete ultrasonic inspections on all fan blades on the some 700 planes in its fleet with CFM56-7B engines over the next two weeks, meeting the FAA’s August deadline by mid-May.

The CFM engine is made by a joint venture of General Electric Co and France’s Safran.

Southwest said it has not found any cracks on fan blades inspected since the accident.