The nose landing gear collapsed after it was lowered to the runway during the landing roll. Examination of the nose landing gear assembly by an FAA maintenance inspector indicated that the nose landing gear actuator was not damaged. It was removed for examination. Disassembly revealed that an extra packing (similar to an O-ring) was installed. This allowed the nose landing gear down and locked light to illuminate but shut the motor off before the nose landing gear was fully extended. Review of the airplane logbooks indicate that a progressive inspection was signed off two days before the accident. During the inspection, it was noted that the nose landing gear actuator switch was leaking. The corrective action indicates that the O-rings were replaced. According to the illustrated parts list of the nose landing gear actuator, two packings are installed. According to the director of maintenance, the mechanic responsible for replacement of the packings was provided with two, but only removed one from the actuator. This resulted in the extra packing. The aircraft had accumulated 4 hours and 6 cycles since the inspection two days earlier.