The U.S. Federal Aviation Administration said it will order inspection of about 220 aircraft engines after investigators found that a broken fan blade touched off an engine explosion this week on a Southwest flight that killed a passenger. NTSB investigators are on scene examining damage to the engine. The regulator said late on Wednesday it plans to finalize the airworthiness directive within the next two weeks. The order, initially proposed in August following a 2016 incident, will require ultrasonic inspection within the next six months of the fan blades on all CFM56-7B engines that have accrued a certain number of takeoffs. Airlines said that because fan blades may have been repaired and moved to other engines, the order would affect far more than 220 of the CFM56-7Bs, which are made by a partnership of France’s Safran and General Electric. The CFM56 engine on Southwest Flight 1380 blew apart over Pennsylvania on Tuesday, about 20 minutes after the Dallas-bound flight left New York’s LaGuardia Airport with 149 people on board. The explosion sent shrapnel ripping into the fuselage of the Boeing 737-700 and shattered a window. Bank executive Jennifer Riordan, 43, was killed when she was partially pulled through a gaping hole next to her seat as the cabin suffered rapid decompression. Fellow passengers pulled Riordan back inside, but she died of her injuries. On Wednesday, National Transportation Safety Board Chairman Robert Sumwalt said the incident began when one of the engine’s 24 fan blades snapped off its hub; investigators found metal fatigue at the break point and a second fracture about halfway along the blade. Sumwalt said the fatigue would not have been observable from the outside. Pieces of the plane were found in rural Pennsylvania by investigators who tracked them on radar. He said this was the first deadly U.S. airline accident since 2009 and that it was too soon to know whether it pointed to a fleet-wide problem with the Boeing 737-700. Southwest was inspecting similar engines in service, focusing on the 400 to 600 oldest CFM56 engines, according to a person with knowledge of the situation; it was the second time this kind of engine had failed on a Southwest jet in two years, prompting airlines worldwide to increase inspections. An NTSB crew was also combing the Boeing 737-700 for signs of what caused the engine to explode. Passengers described scenes of panic after shrapnel from the engine shattered a window and almost sucked Riordan out. Riordan was a Wells Fargo banking executive and a well-known community volunteer from Albuquerque, New Mexico, the company said. Videos posted on social media showed passengers grabbing oxygen masks and screaming as the plane, piloted by Tammie Jo Shults, a former U.S. Navy fighter pilot, prepared to descend into Philadelphia. The airline expected to complete its inspection of the targeted engines in about 30 days. The GE‑Safran partnership that built the engine said it was sending about 40 technicians to help with Southwest’s inspections. Pieces of the engine, including its cowling, were found about 60 miles (100 km) from Philadelphia airport, Sumwalt said. The investigation could take 12 to 15 months to complete. In August 2016, a Southwest flight made a safe emergency landing in Pensacola, Florida, after a fan blade separated from the same type of engine and debris tore a hole above the left wing. That incident prompted the FAA to propose last year that similar fan blades undergo ultrasonic inspections and be replaced if they failed. Edited by David Stamp.