(CBS4) – Worn-out metal could be the reason one of the engines on a Boeing 777 failed and exploded over Broomfield last month. United Flight 328 was thousands of feet above the ground when the incident happened on Feb. 20 shortly after takeoff from Denver International Airport.

(credit: Andy Cross/MediaNews Group/The Denver Post via Getty Images)

Parts of the engine that ripped off are being examined at a hangar at DIA.

On Friday the National Transportation Safety Board said one engine fan blade was cracked, and its surface was consistent with metal fatigue. A second fan blade is also in question.

(credit: CBS)

Both blades were sent to the Pratt & Whitney lab, where the engine was made, to be examined by metal scientists.

After the incident, the plane turned around and returned to DIA with one working engine to make an emergency landing. No one was hurt in the plane or on the ground, where debris fell in residential parts of Broomfield.

(credit: City & County of Broomfield)

The Pratt & Whitney engine had been used on 2,979 flights since its last inspection, the NTSB said. After the incident, the Federal Aviation Administration ordered that certain blades on the same model of engine on other planes be inspected for cracks before their next flight.

The particular Pratt & Whitney engines involved are found on fewer than 100 Boeing 777 jets currently in use. They’re also on several dozen other planes that have been parked because of weak passenger traffic during the pandemic. The affected planes are all used by United Airlines and by carriers in Japan and South Korea.

The engine failure occurred four minutes after the Boeing 777 took off, bound for Hawaii. The NTSB said the pilots had just increased power to the two engines when a loud bang was captured on the cockpit voice recorder.

Information from the plane’s data recorder indicates that the engine shut down automatically and triggered a fire warning.

(© Copyright 2021 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)

Jesse Sarles