The Pratt & Whitney engine that failed minutes into United Airlines flight 328 showed signs of metal fatigue, investigators with the National Transportation Safety Board said during their first public briefing following the incident.

Meanwhile, a third Boeing Co. plane has been involved in a midair incident - this time a Boeing 757 operated by Delta Air Lines flying to Seattle from Atlanta made an emergency landing after an indicator warned of a possible engine problem.

The Delta flight was diverted late Monday to Salt Lake City "out of an abundance of caution," The Seattle Times reported. The plane landed without incident and airport fire crews said the engine didn't appear damaged.

The 16-year-old jetliner was powered by Pratt & Whitney engines, the same manufacturer behind two Saturday engine failures on Boeing planes. Pratt & Whitney is owned by Raytheon Technologies Corp. Its shares were expected to open in New York flat on their Monday close of $73.00 a share - which was a 1.7% fall. Boeing's shares are expected to see a 0.29% increase at Tuesday's open after a 2.11% fall to $212.88 each the previous day.

The share movement follows Boeing's recommendation that all airlines worldwide flying its planes powered by Pratt & Whitney engines remove them from service after an engine failure on a United Airlines flight out of Denver over the weekend. The failure rained aircraft parts onto a suburban neighborhood.

Minutes after taking off and at about 10,000 feet witnesses said they heard a loud boom. Pieces of the United Airlines Flight 328 jet started falling from the sky. Another witness said it looked like "a shower of things were falling out of the sky."

The plane headed back to the airport but continued to drop debris. Photos after the incident showed one home with a hole in its roof. 

British Transport Secretary Grant Shapps said Bowing planes with the same engines will temporarily be taken out of service. Japan Airlines Co. and ANA Holdings Inc. followed suit with 13 and 19 jets grounded, respectively.

South Korea's Asiana Airlines Inc. has stopped operating nine Boeing 777-200 planes.

Boeing said that while 59 of the planes in question are in storage, 69 are in service - sending airlines in a scramble to ground the aircraft.

With Boeing 737 Max jet's issues unravelling over the past years, industry experts and critics have called out the company for slow action in communicating problems to the Federal Aviation Administration and airline operators.

Both the aviation administration and the National Transportation Safety Board are investigating the incident. United Airlines was the first international airline to announce it will remove all Boeing 777 planes from service.

Pratt & Whitney said it was coordinating with world regulators to review engine inspections.

It isn't known if the U.S. incidents have any similarity to another incident this past weekend when a Longtail Aviation Boeing 747 freight plane powered by Pratt & Whitney lost engine parts over the Netherlands' town of Meerssen.

The European Union Aviation Safety Agency said in a statement that, "at this stage" of the investigation, no similarity between the two incidents has been determined.

A spokeswoman for the airport at Maastricht where the 747 freighter took off said that "parts of the engine blade" were seen in photos after the engine failed and exploded. Several homes and vehicles were damaged by the debris and a woman was injured.