Three years before Saturday’s harrowing engine failure and aborted flight to Hawaii, another Boeing 777 headed for Honolulu faced a similar emergency with the same model engine. Both had fractured fan blades and left a trail of debris below, including flying hunks of engine coverings.

Passengers on that February 2018 flight heard a loud bang before their United Airlines jet began to shake violently and pilots struggled to fly with one working engine. Investigators uncovered problems with the inspection process set up by the engine’s manufacturer, Pratt & Whitney, and the Federal Aviation Administration in 2019 ordered airlines to regularly scour fan blades for tiny but potentially catastrophic cracks.

The timing for required inspections was based on how many times engines have been used. The FAA said that “these thresholds provide an acceptable level of safety,” according to the 2019 order.

After independent investigators from the National Transportation Safety Board (NTSB) said two more fan blades broke Saturday — and large metal chunks from the plane were spread over a mile of the Denver suburb of Broomfield — the FAA said it was reconsidering.

“We concluded that the inspection interval should be stepped up,” FAA Administrator Steve Dickson said, noting that a forthcoming emergency order would provide more details.

Aviation experts said investigators will try to determine, among other things, how the failures that preceded the 2018 incident line up with those that led to Saturday’s incident, and whether actions by the FAA, Pratt & Whitney and Boeing to address them were sufficient.

“They’re definitely going to look into this, but they’re not going to initially assume that’s what it is,” said Jeff Guzzetti, who was director of the FAA’s Accident Investigation Division until 2019. “They’re going to have an open mind about other potential failure modes.”

The FAA said Monday that it already had been “evaluating whether to adjust blade inspections” because of a Dec. 4, 2020, fan blade incident on a Japan Airlines jet. That also was a Boeing 777 with a Pratt & Whitney engine, and forced the pilot to return to Okinawa.

A report last month on the Japan incident found that fan blades on the engine were damaged. The nation’s Ministry of Transport ordered domestic airlines to accelerate inspections on jets with similarly built engines by the end of March. These types of inspections are generally done every seven years in Japan.

Japanese regulators announced Sunday that they were grounding the jets. United pulled 24 planes from its fleet that same day, and Boeing recommended air carriers ground 128 of the company’s 777 jets with the same type of engine. South Korean carriers Asiana Airlines and Korean Air announced that they were voluntarily pulling Boeing 777 jets from service, Yonhap News Agency reported.

NTSB Chairman Robert L. Sumwalt said Monday that a preliminary examination of one fan blade in Saturday’s flight “indicates damage consistent with metal fatigue,” though what caused it is unclear. Investigators also have yet to determine when they were inspected. A small piece of blade was found on a soccer field.

Investigators will look at earlier incidents, including the one in Japan, as they work to “find out who knew what when” in the United States and “what could have been done, what should have been done, if anything at all,” he said.

The Denver incident marks the latest safety challenge for Boeing, which recently saw its 737 Max jets returned to the skies after two deadly crashes that killed 346 people. Boeing said it is “actively monitoring recent events related to United Airlines Flight 328.”

Pratt & Whitney said it sent a team to assist the NTSB, which is investigating Saturday’s incident, and coordinating with airlines and regulators on developing new inspection protocols for the engines.

“Pratt & Whitney will continue to work to ensure the safe operation of the fleet,” it said.

Guzzetti said he also is focused on another disturbing pattern highlighted by Saturday’s flight. The terrifying journey, like others in recent years, touches on more than an engine failure caused by broken fan blades, he said. Those are serious breaches themselves. The incidents also point to potential design problems with external structures that house the engines — on 777s as well as other planes — that can be deadly when they shear off, he said.

He pointed to similarities with the case of Jennifer Riordan, a 43-year-old Albuquerque bank executive who died in April 2018 after shrapnel from an engine failure blew out a window on her Southwest 737. A metal latching mechanism, part of the engine-housing structure known as a fan cowl, flew off and smashed against the plane, causing Riordan’s window “to depart,” according to the NTSB.

“There’s a real hazard with engine cowl parts coming off and potentially knocking off the tail, or causing a depressurization, or creating drag when you’re over the ocean and affecting your extended operations,” Guzzetti said.

After investigating the Southwest case, he added, the NTSB pointed out that FAA regulations do not require manufacturers, as part of their designs, to account for where a broken blade could cause the separation of the engine’s outside structure in flight.

“You should not have large pieces of the engine covering, like the inlet and the fan cowl, fall off the airplane because they become projectiles that can damage the airplane, or people on the ground,” Guzzetti said.

Hundreds of Broomfield residents reported finding bits of debris from the United plane, police said. One bit fell through a roof. Another damaged a truck. Images showed incongruously large aircraft parts against homes.

Calls quickly flooded into police.

“911. Are you calling about the airplane incident?” a call-taker began asking, according to audio released by local police.

“Yes, we got debris falling everywhere,” one man said.

“Yeah, we’re getting a ton of calls, thank you,” the call-taker said.

A woman called in to say that a piece of engine almost landed on her head.

“Do you want me to pick this thing up?” she asked.

“No, leave it where it’s at for now, thank you,” the call-taker said.

Dramatic pictures began emerging of the chunks of aircraft engine wedged against suburban homes. A video from aboard the plane posted to social media showed the flayed engine still spinning. By Sunday, the Broomfield police said they were overwhelmed with calls as they tried to recover debris to help federal investigators piece together what happened.

“We are asking the community to only contact us now if they find a large piece of the plane,” the police department tweeted.

On a recording provided by ­LiveATC.net, the pilot of United Flight 328 signaled to air traffic controllers that the plane needed to turn back to Denver.

“Mayday, mayday,” the pilot said in a measured tone. “United Air Flight 328 heavy mayday. Just experienced an engine failure and need to turn immediately.”

Crew members on the jet returned it safely to the ground and no one was hurt.

In all, United has 52 Boeing 777s equipped equipped with engines manufactured by Pratt & Whitney. The airline said it voluntarily removed 24 of them from service Sunday. The remaining 28 are in storage, according to Charles Hobart, a United spokesman.

The airline has an additional 44 Boeing 777 jets that are not affected because they have engines manufactured by GE Aviation.

In a statement Monday, the FAA said that after the 2018 Honolulu flight, the agency “reviewed approximately 9,000 fan blade inspection reports” and issued an Airworthiness Directive that “directed operators to perform initial and repetitive inspections on in-service blades at 6,500 cycle intervals.”

After the Japan Airlines flight last year, the agency reviewed previous inspection records and the engine maintenance history, and “conducted a metallurgical examination of the fan blade fragment to determine the cause of the fracture,” the statement said.

The agency’s “safety experts continue to review all the available data from the investigation of Saturday’s incident,” it said, adding that a new emergency directive will follow the team’s analysis.

Antonios Kontsos, associate professor and director of the theoretical and applied mechanics group at Drexel University’s College of Engineering, said investigators will drill down on why a complex structure designed to operate in an extreme environment and “prevent and contain damage” failed to do so.

Still, the plane’s other systems and skill of the pilot enabled the aircraft to land safely, he said. “We’ll learn from this failure because we’ll examine what went wrong and fix it.”