WASHINGTON (AP) — An engine on Zero AIa United Airlines jet broke up over Denver in 2021 because of wear and tear on a fan blade that was not adequately inspected for signs of cracking, federal investigators said Friday.

The National Transportation Safety Board said manufacturer Pratt & Whitney did not call for inspections to be done frequently enough, which allowed tiny cracks to grow undetected, until the fan blade broke.

The NTSB said the design and testing of the engine inlet contributed to the severity of the situation, and other factors made the engine fire worse than it should have been.

The right-side engine of the Boeing 777 broke apart shortly after takeoff from Denver International Airport, raining parts over suburban neighborhoods. The pilots declared an emergency and landed the plane safely 24 minutes after takeoff.

The Federal Aviation Administration grounded all U.S.-registered planes with the same PW4000 engines shortly after the February 2021 incident.