DENVER (KDVR) — Metal fatigue on a fan blade appears to be the cause of an engine failure on a United Airlines jet that took off from Denver International Airport on Saturday, according to preliminary findings following a National Transportation Safety Board examination.

A MAYDAY call from the Boeing 777-200 was heard shortly after takeoff.

“Our investigators are laying out parts on the hangar floor to closely examine the fractures,” said NTSB chairman Robert L. Sumwalt during a virtual press briefing. “Our mission is to understand not only what happened but why it happened so that we can keep it from happening again.”

Four minutes after takeoff from the airport, a loud bang was heard and vibrations were felt, Sumwalt said.

NTSB investigators discovered two fan blades that were found were fractured. One of the blades indicates damage consistent with metal fatigue, according to the NTSB. The failure led to damage to the underside area of the plane where the wing joins the aircraft body.

“There was no structural damage,” Sumwalt explained.

Aviation experts said much of what dropped from the sky over Denver’s northwest suburbs is known as cowling.

“It’s kind of like, for a lack of a better description, the hood on the car,” said former United contracted pilot instructor Steve Cowell.

The cowling is an outer layer that surrounds aircraft engines.

About 20 minutes after engine failure, United 328 made a successful emergency landing at DIA.

The engine manufacturer is Pratt & Whitney. The fan blade showing potential metal fatigue is being flown to a Pratt & Whitney lab for examination, according to the NTSB. That exam will happen on Tuesday under supervision of federal investigators.