The aircrew of the helicopter contacted McGuire RAPCON and obtained clearance to proceed through the McGuire AFB control zone at 1500 ft. A short time later, the crew declared an emergency. Witnesses on the ground heard a loud 'bang' and observed the helicopter suddenly lose altitude. The helicopter recovered from the descent and began a turn toward RWY 6/24 at McGuire AFB, then it veered sharply to the right and crashed into wooded terrain on Ft Dix. Trees at the accident site were damaged at an 80 degree angle from the horizon. Fire in the area of the engines was extinguished with Halon 1211 and AFFF agents. An exam revealed evidence of a left engine failure. There were indications that fragments from the left engine turbine had impacted the right engine while it (the right engine) was producing power. One of the tail rotor control cables and the tail rotor drive shaft were severed in the area of the right engine. A metallurgical check revealed indications of high cyclic fatigue of the 2nd stage gas producer turbine wheel in the curvic coupling and labyrinth seal area.