What was found
AT 400 FEET AGL AFTER TAKEOFF, CREW HEARD LOUD BANG FROM LEFT (NR 1) ENGINE AND ENGINE RPM WENT TO 'ZERO'. CREW CIRCLED AND MADE SAFE EMERGENCY LANDING. ENGINE WAS REMOVED AND SENT TO MRO FACILITY; DURING ENGINE TEARDOWN, FIVE (5) HP TURBINE ROTOR BLADES WERE FOUND BROKEN IMMEDIATELY UNDER THE BLADE PLATFORM AND SEPARATED FROM THE DISC. OUTER ENGINE CASES WERE NOT PENETRATED. CAUSE OF BLADES' FAILURE IS BEING INVESTIGATED BY ENGINE OEM AND MRO ORGANIZATION ENGINEERING STAFF.