On October 15, 2004, about 0115 Pacific daylight time, a Hughes OH-6A, N6639R, collided with terrain while performing low altitude maneuvers 4 miles west of the Jacumba Airport, Jacumba, California. The US Border Patrol Air Operations was operating the helicopter under the provisions of 14 CFR Part 91. The commercial pilot and a flight crewmember were not injured; the helicopter sustained substantial damage. The local public-use border patrol flight departed Imperial County Airport, Imperial, California, about 2215 on October 14. Visual meteorological conditions prevailed on the dark night, and a flight plan had not been filed. In a written statement, the pilot reported that he was assisting border patrol ground units with a group of illegal aliens located about 4 miles west of Jacumba, in mountainous terrain consisting of tall weeds and thick brush. While maneuvering in the area, the other flight crewmember, an observer, indicated to the pilot that she thought she saw movement in some bushes located behind the helicopter. The pilot maneuvered the helicopter in a turning descent in an effort to provide the observer with a better view of the area. Thinking that the helicopter's tail rotor would be clear of terrain, the pilot maneuvered around the brush. As he turned the helicopter, he heard a loud bang, followed by a vibration, at which point he opted to land. The pilot further stated that after egressing, he completed an inspection of the helicopter. During the examination he discovered that approximately 3 inches of each tail rotor blade were missing; he did not find any other damage. He reported that 3 days after the accident, while inspecting the accident scene, he found red and white paint transfers on a rock, located about 100 yards uphill from the landing site; he noted that the tail rotor blades were painted red and white. During a telephone conversation with a National Transportation Safety Board investigator, the pilot stated that the accident flight was the last flight of his 6-day workweek. He thought that his complacency and eagerness to finish the flight might have contributed to him misjudging the maneuver. He reported no mechanical malfunctions or failures with the helicopter prior to impact.