Authorities have been working "non-stop" after a spectacular engine failure on a US flight that led to dozens of Boeing 777 aircraft being grounded worldwide, the chief air regulator in the US has said.
The fault caused the United Airlines plane bound for Honolulu on Saturday to scatter debris over suburban Denver, although no one was hurt in the air or on the ground.
"We want to understand what happened and take the necessary steps to prevent a similar recurrence from happening," Federal Aviation Administration head Steve Dickson said at an air safety town hall meeting.
"We're fortunate there were no fatalities or injuries," added Mr Dickson, who reiterated that the agency was developing new air worthiness regulations to require stepped-up inspections.
The comments came after investigators pointed to metal fatigue as the chief suspect in the incident -- a fresh setback for Boeing, which only recently resumed deliveries of the long-grounded 737 MAX following two fatal crashes.
It also raises fresh questions about the FAA, which was roundly attacked for its oversight of Boeing in the certification of the 737 MAX, and about whether maintenance was adequate on the plane, aviation experts said.
Even prior to the Denver incident, US air safety regulators had been weighing stricter inspections on the jets and their Pratt & Whitney engines, US officials said.
The FAA reviewed inspection records and maintenance history after a Japan Airlines fan blade incident on December 4 last year "to determine the cause of the fracture and was evaluating whether to adjust blade inspections," an FAA spokesman said.
The flight landed in Japan without injuries.
Following a February 2018 incident on another United jet, the FAA reviewed 9,000 fan blade inspection reports and issued an airworthiness directive setting new rules on inspections.
In a briefing Monday night, the National Transportation Safety Board said it was too soon to know if the issue in Denver was similar to those in the Japan Airlines flight, or the February 2018 incident, which involved another Boeing 777 and Pratt & Whitney engine.
"A preliminary on-scene exam indicates damage consistent with metal fatigue," NTSB Chair Robert Sumwalt told the briefing.
He said two fan blades fractured on the number two engine on the Boeing 777-200 on Saturday.
One of them was later found on a soccer field, while the other remained lodged in the engine.
The NTSB plans also to look at the inspection record on the United plane to see "who knew what when, what could have been done and what should have been done," Mr Sumwalt said.
"Fatigue means that you can have a crack in the material and when you load it again and again, the crack slowly grows," said Robert Kielb, a professor at Duke University's school of engineering.
"This is an example of an event where we learn something about the design 20 years after it goes into service, and then we immediately ground the fleet, figure out what's going on and fix it."
In the wake of the Denver incident, Boeing said all 128 of the 777s with Pratt & Whitney engines were grounded.
Of the 128 planes, only 69 were in service while 59 were in storage.
Besides United, which removed 24 planes from service, affected airlines included Japanese carriers Japan Airlines and All Nippon, and South Korea's Asiana and Korean Air.
On Monday night, a Delta Air Lines flight on a Boeing 757 en route to Seattle from Atlanta was diverted to Salt Lake City "out of an abundance of caution following an indicator warning of a possible problem with one of its engines," a Delta spokesman said.
"The flight landed safely without incident and taxied to the gate without assistance."
The engine failure is unwelcome news for Boeing, which also faces a fresh investigation in the Netherlands after a Boeing 747-400 cargo plane showered a small town with debris, injuring two people, on the same day as the Denver incident.
Boeing only recently resumed deliveries of the 737 MAX following a 20-month global grounding after two crashes killed 346 people.
The MAX began returning to commercial service in late 2020, with airline travel still depressed due to the coronavirus pandemic.
Boeing executives said last month they expected it would take about three years for activity to return to pre-pandemic levels.
Michel Merluzeau, an expert at consultancy AIR, agreed the latest problem did not appear to result from poor plane design.
"It's not really a problem for Boeing," he said. "It's more an issue of maintenance -- how United or Pratt & Whitney is maintaining engines that have been in use for a while."
The episode "is an embarrassing headline, but as a practical issue, it will have no impact on Boeing," said Scott Hamilton of Leeham News, an aviation news site.