DENVER (CBS4) – In a virtual news update on Monday night, the National Transportation Safety Board says they believe metal fatigue played a role in the engine explosion on United Airlines Flight 328. The explosion sent debris falling onto Broomfield neighborhoods and parks.

Some debris was found Monday morning in Arvada.

READ MORE: Exclusive: New Photos Of United Plane Show Hole Torn In Fuselage

(credit: NTSB)

NTSB Chairman Robert Sumwalt says there was a loud bang and vibrations coming from engine number 2 about four minutes after takeoff from Denver International Airport. He says the plane was about 12,000 feet above homes.

Sumwalt says two blades in the engine broke; one fractured at the base where it meets the hub, and the second fractured about mid-way. He says the first blade caused “overload damage” to the second blade.

One of those blades was found on a soccer field.

(credit: NTSB)

New photos obtained by CBS4 – which were taken Saturday after the plane returned to DIA – show a gaping hole in the plane under the right wing, suggesting some of the engine debris struck the plane itself.

READ MORE: FAA Records: United Flight 328 Plane Was Nearly 26 Years Old

(credit: CBS)

Sumwalt also acknowledge damage to the underbody of the aircraft, but says the damage was not structural. The part damaged is a composite fairing that smooths out the plane to make it more aerodynamic.

With regard to the metal fatigue, he says investigators will use what are known as “beach marks” to determine how long the blades had suffered the fatigue, but there are no immediate answers right now.

(credit: NTSB)

Sumwalt says the fire handle in the cockpit was activated and two fire bottles in the engine were discharged. He says the fuel was turned off, and investigators will look into what continued to fuel the fire despite precautions taken.

The engine will be examined which will include a look into its maintenance history. Sumwalt added the agency will compare this event to others in the past.

“Our mission is to understand not only what happened but why it happened so that we can keep it from happening again,” Sumwalt said.

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More than 200 passengers were on board, but nobody was hurt on the plane or on the ground. The plane returned safely to Denver International Airport about 20 minutes after takeoff.

Danielle Chavira