The helicopter emergency medical services (HEMS) pilot stated that he had completed a pre-flight inspection at the beginning of his shift. He noted that earlier in the day, prior to the accident flight, he verified some fuel control settings at the request of one of the mechanics. Both of these required that the engine cowling doors be opened. The pilot stated that prior to the accident flight, he "visually checked for security of doors and cowlings, leaking fluids, tie downs, and overall condition of the aircraft." He reported that after starting the number one engine, the flight medic commented that she heard an "unusual rattle." The pilot subsequently asked her to check the security of the cowling door latches. When the flight medic returned, she informed him that the latches appeared to be secure. The pilot stated that the flight proceeded without incident until about one-half mile from the destination helipad, when he heard a "loud bang and thump from the rear of the aircraft." From that point in time, a "four per rev[olution] vibration had developed." He recalled thinking that due to the noise and vibration something might have separated from the rotor system. He elected to continue the approach to the destination helipad and subsequently landed without further incident. A post accident examination revealed substantial damage to all four main rotor blades. The lower portion of the left engine cowling door was separated. No anomalies were observed with respect to the airframe side of the cowling door latches. The separated portion of the door was not recovered. The helicopter was being repositioned in order to pick-up a patient, and only the pilot and medical crew were on-board at the time of the accident.