On October 23, 2014, about 1400 central daylight time, a Bell model UH-1H helicopter, N72595, was substantially damaged when the left cabin sliding door separated during cruise flight and impacted one of the main rotor blades near San Isidro, Texas. The three crew members were not injured. The helicopter was registered to the U.S. Department of Homeland Security and operated by U.S. Customs and Border Protection under the provisions of 14 Code of Federal Regulations Part 91 as a public use flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The flight originated from the Brooks County Airport (BKS), Falfurrias, Texas, about 1345. The intended destination was the McAllen Miller International Airport (MFE), McAllen, Texas, after conducting a routine law enforcement patrol flight. The crew originally departed MFE about 1100 and completed a routine law enforcement patrol about 2 hours 15 minutes in duration without incident. The flight was operated with the pilot and co-pilot doors, and the main cabin quarter doors removed. The sliding main cabin doors were secured in the fully open position. The crew subsequently landed at BKS in order to refuel and eat lunch. After refueling, the crew conducted a preflight inspection. During that inspection, the co-pilot and tactical team member (TTM), who were inspecting the left side of the helicopter, noted that the open door latch pin was missing from the left sliding door latch. The TTM reportedly discussed the missing pin with the co-pilot. The TTM stated that the co-pilot advised him that the pin was not required to be installed. The co-pilot stated that he had informed the pilot about the missing pin, but he was not sure that the pilot adequately understood him. The crew reported that about 15 minutes after departing BKS, during cruise flight, they heard a loud bang and the pilot felt a momentary upset in the helicopter's flight path. The helicopter performed normally after the event; however, the pilot elected to perform a precautionary landing to an open field. During the post-flight inspection, the crew determined that the left sliding cabin door had departed the aircraft. It was subsequently located separated into two pieces about 1 mile northeast of the landing site. One main rotor blade sustained substantial damage consistent with an in-flight impact with the door assembly. A postaccident examination did not reveal any anomalies consistent with a failure of the door assembly or airframe structure that would have led to the separation. The retaining angle remained attached to the door and appeared undeformed. The door latch pin had separated from the lanyard and was located lying on the aft cabin floor. The exact reason that the pin had separated from the lanyard before the accident flight could not be determined. The helicopter operating handbook permitted flight up to 120 knots with the door properly secured in the open position. In order to properly secure the door, it must be in the full open position with the door latch pin installed. In the event that the door inadvertently opened in-flight, the airspeed should be reduced below 50 knots in order to allow the door to be closed. If the door could not be closed, then it must be fully opened and secured by the open door latch.