The failed engine that forced a plane to make an emergency landing in Philadelphia last month was in compliance with a maintenance recommendation designed to spot flaws like the one that caused the failure, federal authorities reported Thursday.

The information released by the National Transportation Safety Board confirms statements from Southwest Airlines in the wake of the April 17 event, which led to the death of a woman who was partially blown out a window.

On Wednesday, the Federal Aviation Administration issued stringent new testing orders for the part believed to have caused the engine failure, a broken fan blade. The FAA's concern about the possibility of a similar failure prompted the agency to forgo a lengthy public comment process, which usually must happen before a directive is approved.

"The FAA has found that the risk to the flying public justifies waiving notice and comment prior to adoption of this rule, because certain fan blades must be inspected, and, if needed, replaced before further flight," the order states.

The failure on Southwest Flight 1380 happened while the plane with 144 passengers was traveling from New York City to Dallas. The broken fan blade shredded the protective covering around the engine, and debris struck the fuselage and cracked a window. Jennifer Riordan, the woman partially blown from the plane, died from blunt force trauma injuries, a medical examiner found.

>> READ MORE: Twenty minutes of terror in the sky: the emergency landing of Southwest Flight 1380

The incident was the first fatality on an American commercial passenger carrier since 2009.

A similar event happened in August 2016 on another Southwest Boeing 737-700 with the same engine, a CFM56-7B. No one was injured, but the incident led CFM International, the engine's manufacturer, to advise airlines to conduct ultrasonic inspection, a test that allows imaging of the inside of the hollow blades, on any plane that had not had a shop visit in 15,000 cycles. A cycle is the process of starting an engine and shutting it down again and, rather than air miles, is used to measure wear on an engine.

The NTSB also said Thursday that its investigators found within the engine two pieces of the fan blade that failed. The failure appeared to happen near where the blade attached to the turbine, and the NTSB found an area of metal fatigue more than two inches long on the blade. It also found six crack lines in the fatigued area.

The engine involved in the Philadelphia landing last received a maintenance overhaul in November 2012, according to Thursday's announcement. The fan blades were subjected to visual and fluorescent penetrant inspection, a dye test, which would not have spotted fatigue within the blade.

At the time of the emergency landing, the engine had experienced 10,712 cycles since that inspection.  The engine had 32,000 cycles since it was new, the NTSB reported.

>> READ MORE: Trump meets crew of Southwest flight that made emergency landing in Philadelphia

After the 2016 incident, Southwest began conducting eddy current inspections, tests similar to ultrasonic inspections, on planes brought into the shop.

The FAA issued an emergency directive on April 20 requiring a one-time ultrasonic inspection of all fan blades on CFM56-7B engines with more than 30,000 flight cycles since new. On Wednesday, the FAA expanded its requirements, requiring the ultrasonic inspections or eddy current inspections on any engines with 20,000 cycles since new, down from 30,000, or within 113 days of the order's effective date. After that, the FAA recommended the same test be performed every 3,000 cycles since last inspection, roughly a year and a half to two years. Any blades that show signs of fatigue must be replaced.

The order, which goes into effect May 14, will affect 3,716 engines, the FAA stated.

Southwest planned to conduct ultrasonic inspections on every fan blade in its fleet of Boeing 737-700s and 737-800s within 30 days of the Philadelphia emergency landing. The airline did not have specific information on progress, or if the tests had found any other flawed fan blades, but a spokesman said the testing was ongoing.