Boeing Co said it recommended suspending the use of 777 jets with the same type of engine that shed debris over Denver at the weekend after United States regulators announced extra inspections and Japan suspended their use while considering further action.

The moves involving Pratt & Whitney PW4000 engines came after a United Airlines 777 landed safely in Denver on Saturday local time after its right engine failed.

United said the next day it would voluntarily and temporarily remove its 24 active planes, hours before Boeing’s announcement.

Boeing said 69 of the planes were in service and 59 were in storage, at a time when airlines have grounded planes due to a plunge in demand associated with the Covid-19 pandemic.

The manufacturer recommended airlines suspend operations until US regulators identified the appropriate inspection protocol.

The 777-200s and 777-300s affected are older and less fuel efficient than newer models and most operators are phasing them out of their fleets.

Images posted by police in Broomfield, Colorado showed significant plane debris on the ground, including an engine cowling from the 26-year-old plane scattered outside a home.

The National Transportation Safety Board (NTSB) said its initial examination of the plane indicated most of the damage was confined to the right engine, with only minor damage to the airplane.

It said the inlet and casing separated from the engine and two fan blades were fractured, while the remainder of the fan blades exhibited damage.

Japan’s transport ministry ordered Japan Airlines Co Ltd (JAL) and ANA Holdings Inc to suspend the use of 777s with PW4000 engines while it considered whether to take additional measures.

The ministry said that on December 4, 2020, a JAL flight from Naha Airport to Tokyo returned to Naha due to a malfunction in the left engine.

Japan Transport Safety Board said on December 28 that it had found two of the left engine’s fan blades were damaged, one from a fatigue fracture. The investigation is ongoing.

United is the only US operator of the planes, according to the Federal Aviation Administration (FAA). The other airlines using them are in Japan and South Korea, the US agency said.

“We reviewed all available safety data,” the FAA said in a statement. “Based on the initial information, we concluded that the inspection interval should be stepped up for the hollow fan blades that are unique to this model of engine, used solely on Boeing 777 airplanes.”

Japan said ANA operated 19 of the type and JAL operated 13 of them, though the airlines said their use had been reduced during the pandemic. JAL said its fleet was due for retirement by March 2022.

Pratt & Whitney, owned by Raytheon Technologies Corp, said it was coordinating with operators and regulators to support a revised inspection interval for the engines.

An official at South Korea’s transport ministry said it was waiting for formal action by the FAA before giving a directive to its airlines. The US agency said it would soon issue an emergency airworthiness directive.

Korean Air Lines Co Ltd, which has 16 of the planes, 10 of them stored, said on Monday it grounded the remaining six it had in operation of its own volition.

Asiana Airlines Inc, which has nine 777s with Pratt & Whitney engines, said it is in discussion with Boeing and the relevant authorities on what measure to take.

In February 2018, a 777 of the same age operated by United and bound for Honolulu suffered an engine failure when a cowling fell off about 30 minutes before the plane landed safely. The NTSB determined that incident was the result of a full-length fan blade fracture.

Because of that 2018 incident, Pratt & Whitney reviewed inspection records for all previously inspected PW4000 fan blades, the NTSB said. The FAA in March 2019 issued a directive requiring initial and recurring inspections of the fan blades on the PW4000 engines.