The National Transportation Safety Board stated on Friday that the United Airlines Boeing 777 engine failure in Colorado in February 2021 was caused by a crack in a fan blade and pointed to insufficient inspections as a contributing factor.

The Federal Aviation Administration (FAA) immediately required examinations of 777 aircraft with Pratt & Whitney 4000 engines prior to further flights following the accident, which resulted in the planes’ being grounded for more than a year.

The Boeing 777-200 bound for Honolulu after takeoff from Denver showered debris over nearby cities, but no one was injured and the plane safely returned to the airport.

The NTSB cited “the inadequate inspection of the blades, which failed to identify low-level indications of cracking, and the insufficient frequency of the manufacturer’s inspection intervals, which permitted the low-level crack indications to propagate undetected and ultimately resulted in the fatigue failure.” Pratt & Whitney, a unit of RTX, did not immediately comment.

United said on Friday it “closely collaborated with the NTSB, FAA, Boeing and Pratt and Whitney on each step of the investigation and are pleased to have these aircraft back in our fleet.”

In March 2022, the FAA finalized new safety directives after three reported in-flight fan blade failures including the Colorado incident that prompted enhanced inspections and modifications. The FAA said on Friday it had issued the safety directives in response to the fan blade incidents.

Boeing said since the failure it “has identified appropriate design changes to improve the structural integrity of the engine inlet and cowling and has been communicating with the FAA, Pratt & Whitney and airline customers on its progress.”

United is the only U.S. operator of 777s with the PW4000 engine and had 52 of those planes as of 2022.

As of January, 17 confirmed cracked fan blades have been found, the NTSB said, the first of which was identified in December 2004 – not including three fan blades that sustained full-blade separation in service.