On January 3, 2005, at 1346 mountain standard time, a Eurocopter EC135T1 helicopter, N135NW, sustained a loss of control in flight and collided with terrain at Casa Grande, Arizona. The flight was operated by Omni Flight Helicopters, Inc., d.b.a. Native American Air Ambulance, under the provisions of 14 CFR Part 91. The helicopter was substantially damaged. The certificated commercial check pilot received a minor injury and the certificated airline transport pilot was not injured. Visual meteorological conditions prevailed, and a company flight plan was filed. The local area training flight originated at Mesa, Arizona, about 1330. The pilots reported that as they turned base in the traffic pattern for landing at the airport, the helicopter pitched up violently and rolled to the right, followed by a hard roll to the left. Both pilots got on the controls and tried to regain control of the helicopter. The pilots lowered the collective and started down for a landing. During the descent, the pitching and rolling continued intermittently with the roll attitude reaching beyond 60 degrees. As the helicopter neared the ground, the pilots flared and the helicopter hit the ground in a tail low attitude and rolled over. A post accident examination of the helicopter, and examination of maintenance records by a Federal Aviation Administration inspector, occurred on site and after recovery. During the course of the examination it was determined that maintenance personnel had removed cyclic control links from the cyclic control levers to check the ball joints for excess free play during an inspection. The links are attached to the cyclic levers with bolts; castle nuts with cotter pins through the castle nuts as a safety locking device. The right lateral cyclic control link was found broken. The nut on the upper bolt was 1/4-inch away from the side of the attachment point. There was no evidence of the cotter pins having been added to the castle nuts after the link inspection/reinstallation.