The National Transportation Safety Board (NTSB) investigation into the accident attempted to reconstruct the aircraft's performance based on reports from witnesses and survivors. Witnesses reported that the plane attempted to climb at a higher than normal angle, and that the plane rolled from side to side and appeared to be out of control. Based on examination of the wreckage, the NTSB determined that collision with the instrument landing system (ILS) localizer antenna array approximately beyond the departure end of the runway 18L led to the breakup of the aircraft. Leaking jet fuel started a fire that quickly engulfed the fuselage. Engine #3 had separated from the empennage. Passengers reported that the plane appeared to shake violently on takeoff. Witnesses on the ground claimed that one of the engines was on fire, but although the engine was badly crushed from ground impact, there was no sign of fire or heat damage. Delta officials quickly reacted to questions about the 727's operating and maintenance condition by pointing out that the original factory engines from 1973 (when the plane was initially purchased by the airline) had been replaced in the last few years by a newer model that ran more quietly and used less fuel. They also stated that there were no known mechanical or maintenance problems with the plane prior to the flight (the vibration reported by passengers was the compressor surging mentioned above).