Full Report:

A helicopter with a pilot, a hoist operator, and a helicopter rescue specialist (rescuer) on board was dispatched to transport an injured person out of an area of rough terrain. Based on the patient's location and the time of the call, a night hoist operation was planned. The helicopter arrived on scene, and the hoist operator lowered the rescuer and equipment from the helicopter. While the rescuer and ground personnel prepared the patient for transport, the pilot and hoist operator looked for a nearby landing zone. Upon hearing radio calls from the rescuer that the patient was ready, the helicopter returned to the patient's location, and the hoist operator lowered the hoist hook. The patient, who was in a Bauman bag, and the rescuer were then lifted from the ground by the hoist. The hoist operator continued to reel in the patient and the rescuer as the helicopter transitioned from a hover to forward flight. When the patient and rescuer were about 10 ft below the helicopter's skids, the rescuer fell about 100 ft to the ground.
The ground personnel who helped the rescuer prepare the patient for transport did not report seeing anything unusual. They said that, during the initial part of the lift, the rescuer and patient went into some tree branches, the helicopter maneuvered away from the tree, and then the rescuer and patient were lifted up towards the helicopter.
Examination of the rescuer's equipment did not reveal any failures or malfunctions that would explain the fall. Additionally, examination of the hoist hook and helicopter equipment did not reveal any abnormities. Also, review of video from a camera located on the hoist did not identify any failures in the equipment nor did it show how the rescuer was initially hooked into the hoist. In the absence of any equipment failure, it is likely that the rescuer was not properly fastened to the hoist.
Probable Cause and Findings
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The helicopter rescue specialist was not properly attached to the hoist system, which resulted in a fall during a night hoist operation.