Following the fatal engine failure on a 737 Tuesday, the Federal Aviation Administration ordered emergency inspections of similar CFM56-7B engines manufactured by CFM International within 20 days.

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U.S. aviation regulators ordered emergency inspections of one of the world’s most popular jet engines after the fatal accident this week on a Southwest Airlines plane.

The Federal Aviation Administration order covers an estimated 352 CFM International engines in the U.S. that have made at least 30,000 flights, the agency said in a statement Friday. The emergency order is effective immediately, and inspections must be completed within 20 days, the FAA said.

The European Aviation Safety Agency is adopting similar requirements, said a person familiar with the matter. There are 681 of the engines worldwide and regulators in other nations generally follow the FAA’s lead.

The FAA order adds to the scrutiny of the CFM56-7B, the type of engine that blew out on Southwest’s Boeing 737 on Tuesday. Investigators say a fan blade broke off in flight, triggering a chain of events that shattered the window on the 737-700.

Passenger Jennifer Riordan, who was a vice president at Wells Fargo in New Mexico, was killed after being partially sucked out of the opening. The plane made an emergency landing in Philadelphia.

Southwest has sent a letter of apology, a $5,000 check and a $1,000 travel voucher to passengers who were on the flight.

“Please accept our deepest apologies,” Chairman Gary Kelly wrote in a letter dated April 18, a day after the accident.

“We value you as our customer and hope you will allow us another opportunity to restore your confidence in Southwest as the airline you can count on for your travel needs,” Kelly wrote. “In this spirit, we are sending you a check in the amount of $5,000 to cover any of your immediate financial needs.”

The FAA said it ordered the accelerated inspections because metal fatigue in a fan blade “is likely to exist or develop in other products of the same design.”

Engine maker CFM, a joint venture between General Electric and Safran, issued a service bulletin recommending stepped-up checks because the fan blades on the engine that failed on Tuesday wouldn’t have been covered for immediate inspections under the earlier standards.

Engines with at least 20,000 flights — or what the industry refers to as “cycles” — must be checked for cracks by the end of August, CFM said. After the fan blades are inspected on the engines, the company said it recommends that they be reexamined after 3,000 flights, which equals about two years of airline service.

The order would’ve required an inspection on the engine that failed Tuesday, said the person familiar with the action, who wasn’t authorized to speak. CFM had recommended airlines first inspect engines with 15,000 flights or more, but the engine involved in Tuesday’s failure had only 10,000 since its last overhaul.

The incident raised a number of questions because jet engines are certified to be able to withstand a broken fan blade without causing major damage.

The manufacturer had issued two service bulletins last year calling for additional inspections of fan blades on the CFM56-7B engines following a similar incident in 2016 on another Southwest plane.

In the earlier case, a fan blade fractured and broke loose, bouncing in front of the engine’s protective cover and then striking the plane, causing it to lose pressure. No one was hurt; the plane made an emergency landing.

The FAA last August notified carriers it was prepared to require inspections based on CFM’s service bulletins, but hadn’t finalized those inspections. The agency announced Wednesday it would issue those orders within weeks.

Out of about 14,000 CFM56-7B engines in operation around the world, about 680 will be subject to the inspections within 20 days, the company said in the release. More than 150 of those have already been checked by operators, it said.

Inspections can be conducted without having to dismantle an engine and it takes about four hours per engine. CFM recommends that airlines use an ultrasound device, which can detect small cracks beneath the surface using sound waves.

Southwest, American Airlines Group, Delta Air Lines and Alaska Airlines all began inspections of 737 engine fan blades last year, after the 2016 Southwest incident, even though a proposed FAA requirement hadn’t been finalized, the carriers said.

Delta and Southwest extended their inspections to other 737 engines voluntarily. American has finished inspections of its most-flown engines and is working on the rest.

Alaska said Friday that it has no engines with more than 30,000 flights, and that its policies after the first inspection exceed the manufacturer’s requirements by mandating repeated ultrasonic inspections “every 1,850 flight cycles or 5,000 flight hours, whichever comes first.”

United Continental Holdings Inc. said Thursday it is following a service bulletin issued to it last week for its 698 engines, and expects the work to continue through the year.