A piece from the damaged Southwest jet was found in Bernville, Pennsylvania, 70 miles outside of Philadelphia, NTSB Chairman Robert Sumwalt said at a press conference tonight.

The engine cowling was discovered in an area near where the crew first observed issues with the aircraft, he said. Investigators performed a preliminary examination of the engine and found evidence of metal fatigue, where the blade separated from the engine, Sumwalt said.

A preliminary review of the aircraft's voice recorder indicated Southwest flight 1380 was initially "routine," he said. About 20 minutes after takeoff, multiple warnings and alerts went off in the cockpit, Sumwalt said.

The crew deployed their oxygen masks and reported a fire in the No. 1 engine, he said. They later determined there was no fire.

"They were operating single engine and they were initiating an emergency descent," Sumwalt said.

The crew requested emergency personnel assistance and an extended final approach "because they were concerned with potential aircraft controllability issues," he said.

Sumwalt said specialists will be examining the aircraft, engine and maintenance records.