(Reuters) - The U.S. Federal Aviation Administration said it will order inspections of at least 220 aircraft engines as investigators are focusing on a broken fan blade in an engine that exploded Tuesday on a Southwest Aiirlines flight, killing a passenger.

The regulator said late on Wednesday it plans to finalize the air-worthiness directive within the next two weeks.

The order, which it initially proposed in August following an engine failure in 2016 on another Southwest Airlines Co flight, will require ultrasonic inspection within the next six months of fan blades on all CFM56-7B engines that have accrued a certain number of takeoffs, while others will need inspections within 18 months.

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.

An FAA official acknowledged the total number covered may be higher. Some critics have questioned why the FAA did not move faster, when European regulators issued an April 2 directive ordering similar CFM56-7B inspections within nine months.

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 plane and shattered a window.

Bank executive Jennifer Riordan, 43, was killed when she was partially pulled through the gaping hole as the cabin suffered rapid decompression. Fellow passengers were able to pull her back inside but she died of her injuries.

Videos posted on social media showed passengers grabbing for oxygen masks and screaming as the plane, piloted by Tammie Jo Shults, a former U.S. Navy fighter pilot, prepared for the descent into Philadelphia.

Shults and the first officer in a joint statement declined media interviews. “We all feel we were simply doing our jobs. Our hearts are heavy,” the pair said.

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On Wednesday, National Transportation Safety Board (NTSB) Chairman Robert Sumwalt said the incident began when one of the engine’s 24 fan blades snapped off from its hub. Investigators found that the blade had suffered metal fatigue at the point of the break.

Sumwalt said he could not yet say if the incident, the first deadly passenger airline accident in the United States since 2009, pointed to a fleet-wide problem in the Boeing 737-700.

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Southwest crews were inspecting similar engines the airline had in service, focusing on the 400 to 600 oldest of the 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 the past two years, prompting airlines around the world to step up inspections.

A NTSB inspection crew was also combing over the Boeing 737-700 for signs of what caused the engine to explode.

Investigators expected to remain on the scene into the weekend but the NTSB did not plan any media update on Thursday. The NTSB investigation could take 12 to 15 months to complete, Sumwalt said.

Sumwalt said the fan blade, after suffering metal fatigue where it attached to the engine hub, has a second fracture about halfway along its length. Pieces of the plane were found in rural Pennsylvania by investigators who tracked them on radar. The metal fatigue would not have been observable by looking at the engine from the outside, Sumwalt said.

The GE-Safran partnership that built the engine said it was sending about 40 technicians to help with Southwest’s inspections. Southwest said it expected to wrap up its inspection of the engines it was targeting in 30 days.

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 ripped 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.