What's to blame?

The cause of the engine failure was traced back to a manufacturing defect in the fan disk which had microscopic cracks due from flying the plane over 15 years. The titanium fan disk crack grew a little bigger each flight. The crack grew to 13 millimeters and was present during maintenance inspections. The crack should have been detected by maintenance crew but never was. The picture below shows the recovered fan disk that split into two piece during flight.