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FAA directs air carriers to inspect engines similar to the one that failed on the United Airlines flight

Hayden A. Smith, 17, is a plane spotter and photographer and took this photo of UAL Flight 328 on Saturday, February 20, as it flew overhead in Aurora, Colorado.

(CNN)The Federal Aviation Administration on Tuesday directed air carriers to inspect all Pratt & Whitney engines similar to the one that dramatically failed on a United Airlines flight on Saturday.

The directive effectively grounds the Boeing 777-200 planes that use this specific PW4000 series engine, but the only US operator with this type of engine in its fleet, United Airlines, already said Sunday that it would immediately pull 24 planes from flying "out of an abundance of caution."

The FAA called for carriers to conduct a special thermal acoustic image (TAI) inspection because the tiny cracks in the fan blades that indicate wear and tear are not necessarily visible to the naked eye.

"TAI technology can detect cracks on the interior surfaces of the hollow fan blades, or in areas that cannot be seen during a visual inspection," it said.

Pratt & Whitney said in a statement that the process requires the shipment of the fan blades to the company so that TAI inspection can take place.

"Pratt & Whitney is coordinating all actions with Boeing, airline operators and regulators. The safe operation of the fleet is our top priority," their statement said.

The FAA said it will "review the results on a rolling basis," adding it may revise how often these engines are inspected.

An investigation is ongoing

National Transportation Safety Board (NTSB) investigators said earlier this week the Pratt & Whitney engine that failed minutes into United Airlines flight 328 showed signs of metal fatigue.

NTSB Chairman Robert Sumwalt said on Monday a preliminary on-scene investigation indicated damage "consistent with metal fatigue" of one fan blade in the plane's right engine. Investigators said that blade broke free at the hub, likely hitting another that was broken mid-span.

A section of a blade was slated to be examined on Tuesday in a Pratt & Whitney lab. Investigators believe they can determine for how long the blade was damaged before it failed.

The PW4000 engine in question had 22 blades, investigators said, adding that they found one lodged inside in the jet engine's containment ring while another was found in a soccer field in Broomfield, Colorado -- along with a mile-long trail of debris that authorities also found in the area.

None of the passengers were injured, nor were residents in Broomfield.

The flight fuselage also had damage to a non-critical composite piece designed to make the plane more aerodynamic, Sumwalt said.

CNN's Amir Vera contributed to this report.