On September 22, 1995, about 0750 Atlantic standard time, a Bell 206B, N2280N, registered to Hill Construction Company, and operated by the U.S. Department of the Interior, ditched in the Atlantic Ocean near Culebra, Puerto Rico, following a loss of tail rotor control, while on a 14 CFR Part 91 public use flight. Visual meteorological conditions prevailed at the time and a company flight plan was filed. The aircraft received substantial damage and the airline transport-rated pilot received minor injuries. The three passengers received serious injuries. The flight originated from San Juan, Puerto Rico, the same day about 0720. The pilot stated that while in cruise flight at 800 feet, near the island of Culebra, en route from San Juan to St. Johns, U.S. Virgin Islands, he felt a high frequency "shudder and growl." He turned toward Culebra in preparation for a landing. After turning toward the island the shudder increased and the helicopter's nose yawed to the right. He reduced engine power and increased airspeed in an attempt to control the helicopter. Upon descending to 400 feet he was not able to control the right yaw and he entered autorotation. In autorotation the right yaw stopped and while descending he activated the emergency floats. The helicopter touched down on rough water, rolled on to the right side, and began to sink. He and the passengers exited the helicopter and were picked up by a boat. The pilot of another helicopter, operated by Hill Aviation, flew over the crash site shortly after the accident. The pilot stated he exchanged hand signals with the pilot of N2280N and determined the pilot and three passengers were alright. He observed in his fly over that N2280N's tailboom was severed in the area of the horizontal stabilizer. The helicopter was examined after recovery by FAA inspectors and representatives of the Department of the Interior and Bell Helicopter. Continuity of the engine, transmission, main rotor, and tail rotor drive was confirmed. The aft portion of the tailboom, including the tail rotor drive shaft, tail rotor gearbox, and tail rotor was missing during this examination. The last section of tail rotor driveshaft that remained with the helicopter was removed and forwarded to Bell Helicopter for examination by Bell Helicopter and FAA personnel. The shaft was identified as the number 3 tail rotor driveshaft. The shaft failed as a result of bending overstress. See Bell Helicopter report. About 2 weeks after the accident the aft portion of the tailboom was found. The tail rotor gearbox and tail rotor were not attached to the tailboom when it was found. Examination of the aft tailboom was performed at Bell Helicopter, Fort Worth, Texas by FAA inspectors and representatives of Bell Helicopter. The tailboom separation was the result of a main rotor blade strike. "Examination of the assembly revealed evidence that the gearbox was loose for some extended period of time. Wear and deformation had occurred where the tail rotor gearbox was in contact with the top mounting surface of the tailboom and inside the gearbox fairing. The tail rotor driveshaft thomas coupling at the forward end of the gearbox had been moving out of its normal position causing tearing and gouging of the tailboom and wear and deformation inside the gearbox fairing. This indicated the tail rotor driveshaft was rotating during and before final separation of the tail rotor gearbox assembly from the tailboom. The tailboom separation appeared to be the result of a main rotor blade strike." See the Bell Helicopter report. Examination of aircraft records showed the tail rotor gearbox input seal was changed on June 30, 1995, 110 flight hours before the accident. Bell Helicopter representatives stated the tail rotor gearbox must be removed from the helicopter to perform this seal change. On September 18, 1995, 14 flight hours before the accident a 100 hour inspection was performed. One item on the inspection checklist, that was signed off by a mechanic, is to check the torque of the tail rotor gearbox retaining nuts. See the logbook records. Bell Helicopter operating procedures for the Bell 206B require that the pilot check the condition of the tail rotor and tail rotor gearbox during preflight inspection. The helicopter was registered to Hill Construction Company and operated by Hill Aviation, Inc., a 14 CFR Part 135 Air Taxi Operator. At the time of the accident the helicopter was being flown under a U.S. Department of the Interior Basic Ordering Agreement (BOA) between Hill Aviation, Inc. and the Department of the Interior. On May 5, 1997, a representative of Bell Helicopter, wrote a letter to the NTSB IIC for this investigation, stating that another Bell Helicopter representative's previous statement to the NTSB that the tail rotor gearbox had to be removed from the helicopter to replace the input seal on June 30, 1995, was not correct. The representative state that the tail rotor input seal can be replaced while the gearbox is still mounted to the tailboom. See attached letter and page from the Bell 206B Maintenance Manual.