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DENVER (KDVR) — The NTSB reports one of the engine fan blades on the United flight that rained debris over Broomfield last month showed secondary fractures under a microscopic observation.

The engine experienced damage approximately four minutes after takeoff when the crew and passengers heard a loud bang from the second engine on the plane.

The flight deck recorder data indicated the engine made an unordered shutdown and the engine fire warning activated shortly after.

As part of a standard engine fire checklist, the crew discharged two fire bottles into the engine but the indicator did not turn off until the plane was downwind for landing.

According to the investigation report, “The flight crew continued to prepare for the emergency landing by completing additional critical checklists and verifying airplane performance for landing. They elected not to dump fuel for safety and time reasons and determined that the magnitude of the overweight landing was not significant enough to outweigh other considerations. The captain accomplished a one-engine-inoperative approach and landing to runway 26 without further incident.”

Fire and emergency crews met the plane on the ground and immediately hit the engine with water and foam to extinguish the fire completely.

Officials report the valve that controls fuel to the engine was found closed and determined not to be a fuel-fed fire. Multiple broken fuel, oil, and hydraulic lines were found in an examination of the engine accessories and the gearbox was fractured.

Officials say the fan blade under investigation was inspected in 2014 and 2016 and the report of the inspection from 2016 was re-examined after an incident with another Pratt & Whitney engine in 2018.

Authorities continue to investigate the maintenance records to examine the presence and disposition of any anomalies data near the fracture point of the blade.