The Federal Aviation Administration on Saturday ordered a temporary grounding of dozens of Boeing 737 Max 9 aircraft for inspections, a day after a piece of the aircraft blew out in the middle of an Alaska Airlines flight.

Images and video of Alaska Airlines Flight 1282 that were shared on social media showed a gaping hole on the side of the plane and passengers using oxygen masks before it returned to Portland shortly after taking off for Ontario, California, on Friday afternoon.

The FAA’s emergency airworthiness directive will affect about 171 planes worldwide and applies to U.S. airlines and carriers operating in U.S. territory, the agency said. Alaska and United Airlines said late Saturday that they were grounding their entire fleets of Boeing 737 Max 9s.

No serious injuries were reported on the flight, according to federal safety officials. There were 171 passengers and six crewmembers on board, Alaska Air said.

“Safety will continue to drive our decision-making as we assist the NTSB’s investigation into Alaska Airlines Flight 1282,” FAA Administrator Mike Whitaker said in a statement.

Large-scale groundings of aircraft by the FAA or other aviation authorities are rare. The FAA has heavily scrutinized the Boeing 737 Max since two fatal crashes grounded the jetliner worldwide almost five years ago. Two other models of the Max, the smallest and largest version, have not yet been cleared by the agency to enter commercial service.

The section of the fuselage missing appeared to correspond to an exit not used by Alaska Airlines, or other carriers that don’t have high-density seating configurations, and was plugged.

The National Transportation Safety Board has started its investigation. Chair Jennifer Homendy, at a press briefing in Portland Saturday night, asked the public for help in finding the plane’s missing door.

Homendy said no passengers were seated at the seat closest to the panel or the middle seat in the row where the door blew out and added that it was fortunate that the plane was still climbing and not at cruising altitude when travelers and crew could have been standing or walking through the cabin.

“We could have ended up with something more tragic,” she said.

The incident was described as “an explosive decompression at the window exit,” according to Sara Nelson, president of the Association of Flight Attendants-CWA, the labor union that represents Alaska’s cabin crew and flight attendants at United, Spirit and other carriers.

Anthony Brickhouse, a professor of aerospace safety at Embry-Riddle Aeronautical University, said such an incident is extremely rare.

“Rapid decompression is a serious matter,” he said. “To see a gaping hole in an aircraft is not something we typically see. In aviation safety, we would call this a structural failure.”

The incident is also a reminder to keep your seatbelt fastened when seated, he added.

“I always advise people on a commercial aircraft, keep your seatbelt on regardless of what the light says,” Brickhouse said.

Before the FAA issued its directive, Alaska Airlines earlier said it would ground its fleet of Boeing 737 Max 9 planes. On Saturday, the carrier said 18 of the planes “had in-depth and thorough plug door inspections performed as part of a recent heavy maintenance visit,” but later said it would temporarily ground them all.

“We are in touch with the FAA to determine what, if any, further work is required before these aircraft are returned to service,” Alaska said.

As of 7 p.m. ET, Alaska said it canceled 160 flights, affecting 23,000 customers.

The National Transportation Safety Board sent a team to Portland on Saturday to investigate the incident.

United Airlines, the largest operator of the planes in the U.S., had prepared to ground dozens of its Boeing 737 Max 9 aircraft for inspections, CNBC reported earlier. The carrier said late Saturday that it had grounded its entire fleet of 79 Boeing 737 Max 9 aircraft, after earlier saying 30 of the planes had already satisfied the FAA’s inspection requirement.

The FAA said the inspections will take between four and eight hours per plane.

The Boeing 737 Max 9 is a larger version of Boeing’s best-selling jetliner, the 737 Max 8. Max planes were grounded worldwide in 2019 after two fatal crashes within about five months of one another. The U.S. lifted its flight ban on the jets in late 2020 after software and training updates.

The Boeing 737 Max 9 has an emergency exit door cut behind the wings for use in dense seating cabin configurations, like those used by budget airlines, according to Flightradar24.

“The doors are not activated on Alaska Airlines aircraft and are permanently ‘plugged,'” Flightradar24 said.

Boeing didn’t comment beyond its statement when asked about the sealed emergency exit door. Spirit AeroSystems, which makes the fuselages for the planes, referred CNBC to Boeing when asked about the incident.

“Safety is our top priority and we deeply regret the impact this event has had on our customers and their passengers,” Boeing said in a statement on Saturday. “We agree with and fully support the FAA’s decision to require immediate inspections of 737-9 airplanes with the same configuration as the affected airplane.”

The company said it is supporting the NTSB’s investigation.

There are 215 Boeing 737 Max 9 planes in service worldwide, according to aviation-data firm Cirium. In addition to United and Alaska Air, other operators include Aeromexico, Turkish Airlines, Icelandair and Panama’s Copa Airlines.

Southwest Airlines and American Airlines operate the smaller 737 Max 8.

Late last year, Boeing urged airlines to inspect aircraft for a “possible” loose bolt in the rudder control system, the latest in a series of manufacturing flaws on Boeing jets that have prompted additional inspections, and slowed deliveries of the jets.