Accident Sequence
During the takeoff roll, the flightcrew of N425EX experienced a 'thump' that resounded through the airplane. Concurrently, they observed erratic engine indications from the n°3 engine. The captain aborted the takeoff and brought the airplane to a stop on the runway. After the airplane came to rest, the fire warning system activated on the n°3 engine. The crew attempted to extinguish the fire using the airplane's fire extinguishing system, but their efforts were unsuccessful.
Post-Accident Examination
The post-accident examination of the n°3 engine revealed that the high-pressure compressor disk had failed due to a fatigue crack that originated from a corrosion pit. The shrapnel from the disk penetrated the engine nacelle and severed the main fuel line for the engine. The released fuel ignited and burned into the fuselage, igniting the 12,600 lbs of cargo being transported (U.S. Mail).
Findings
The official probable cause of the accident was a catastrophic and uncontained failure of the n°3 engine, which resulted in a fuel fed fire.