The aircraft impacted vertically in a heavily wooded swamp. Evidence indicated that the aircraft impacted with a high vertical rate of descent and low rotor RPM. Investigation revealed that the belt drive clutch engagement cable had separated due to abrasion caused by internal contact between the cable and assembly components. Separation of the cable resulted in a disconnect between the engine and the transmission. Review of the aircraft maintenance records could not determine the exact total time on the cable; however, the records did indicate that the cable had last been disassembled and inspected about 636.8 hours before the accident. A manufacturer's service information notice had been issued on the cable assembly which required an initial inspection within 50 hours and a recurring inspection every 400 hours thereafter, until the unit was replaced with an improved model. The aircraft maintenance records did not indicate compliance with the information notice; however, the operator stated that the inspections were being performed.