Metal fatigue may have caused an engine explosion during a United Airlines flight over Denver on Saturday, investigators say.
The Pratt & Whitney PW4000 engine caught fire shortly after take-off and debris rained down on a residential area below.
A total of 231 passengers and 10 crew were on board the Boeing-777 aircraft, which was bound for Honolulu, but nobody was hurt and the plane landed safely.
United Airlines, Egyptair and operators in Japan and South Korea, have all since said they will not fly some of their Boeing planes until further inspection.
Boeing has also recommended that airlines suspend use of the planes while the US Federal Aviation Administration (FAA) identifies an appropriate inspection protocol.
The National Transportation Safety Board (NTSB) revealed last night that a preliminary assessment showed two fan blades on the Denver plane's engine had broken.
One blade had signs of metal fatigue, while investigators believe the other was chipped when the first broke off.
According to NTSB chairman Robert Sumwalt, the blades are being sent to the Pratt & Whitney laboratory for review under the supervision of NTSB inspectors.
"Our mission is to find out what happened and why it happened, so we can keep it from happening again," he said.
The Denver explosion is being linked to another serious engine failure, which occurred on a United Airlines flight from San Francisco in 2018. Then, the right engine broke shortly before the plane landed in Honolulu.
Like the incident on Saturday, an investigation by the NTSB found the failure was caused by a full-length fan blade fracture.
The FAA has said it plans to issue an emergency airworthiness directive soon that will require stepped-up inspections of the fan blades for fatigue.
Pratt & Whitney did not respond immediately to a request for comment.