Damaged fan blade from United engine failure showed signs of metal fatigue

One of the fan blades that was fractured in the Pratt and Whitney engine that failed during United Airlines Flight 328 showed signs of "metal fatigue," National Transportation Safety Board (NTSB) officials said Monday night.

The blade is being flown on a private jet to the manufacturer to undergo examination Tuesday.

United 328 experienced an engine failure shortly after takeoff from Denver on Saturday and the pilots conducted an emergency landing. Debris from the plane was strewn across multiple neighborhoods, but there were no injuries on the plane nor on the ground.

"Our mission is to understand what happened and why it happened so that we can keep this from happening again," NTSB Chairman Robert Sumwalt said.

Two of the 22 fan blades on the Pratt and Whitney engine were fractured in the incident, the NTSB said. One blade was fractured at the root and the other adjacent blade was fractured at mid-span.

"Basically, it got hit from the other piece that was separated," Sumwalt said.

Chairman Sumwalt could not share when this aircraft or engine was last inspected or how often it required inspection.

The FAA issued an Emergency Airworthiness Directive on Sunday, calling for "immediate or stepped-up inspections" on Boeing 777s powered by the same Pratt and Whitney engine that failed during United Airlines flight 328.

Boeing said it supported the grounding of all 128 of its 777 jets powered by the particular Pratt and Whitney engine model until "the Federal Aviation Administration identifies the appropriate inspection protocol."

United Airlines temporarily removed the affected 777s from its schedules -- amounting to 52 aircraft. More than 100 Boeing 777s are now grounded worldwide.

Pratt and Whitney said it is "actively coordinating with operators and regulators to support the revised inspection interval of the Pratt & Whitney PW4000 engines that power Boeing 777 aircraft." The company also said it has dispatched a team to work with investigators.

The NTSB usually releases a preliminary report within two weeks of an incident, but the final report could take up to 18 months.

ABC News' Sam Sweeney contributed to this report.