On February 23, 2002, about 1600 eastern standard time, a Piper PA-24-260, N9055P, was substantially damaged while landing at the Atlantic City Municipal/Bader Field (AIY), Atlantic City, New Jersey. The certificated private pilot and passenger were not injured. Visual meteorological conditions prevailed for the personal flight that departed Winchester, Virginia, destined for Bader Field. A visual flight rules flight plan was filed, and the flight was conducted under 14 CFR Part 91. According to the pilot, once in the local area he contacted Atlantic City Approach, and then changed over to Bader Field's common traffic advisory frequency (CTAF). He saw two helicopters circling over a submerged airplane that had just ran off the end of the runway at Bader Field and into the water. After entering the traffic pattern, the pilot turned final for runway 11, and announced his intentions. He then saw an airplane holding short for the same runway he was planning on using, so he made another announcement. When the pilot was approximately 100 to 150 feet agl, the airplane holding short pulled onto the runway. The pilot executed a go-around. He retracted the flaps and landing gear, and then did a left 360-degree turn, placing the airplane back on final. The airplane touched down and started to roll out. When it was approximately 200 feet from the end of the runway, the landing gear retracted, and the airplane slid approximately 100 feet before coming to a stop. After the accident, the pilot realized the landing gear switch was in the up position. He believed he raised the gear inadvertently during roll out, but did not remember doing it. He was sure he landed with the gear down, but added, "If the gear retracted while on the ground, the gear doors should have been damaged or scratched," which they were not. According to a mechanic that examined the airplane, there was no damage to the landing gear doors, or side scuffing on either tire. In addition, no damage was observed on either flap. The airplane was raised in order to lower the landing gear. The gear was extended, and locked in the down position. A crack was identified in the landing gear actuator. It did not affect the down locks, and the mechanic thought it was caused when the airplane was recovered. The keel of the airplane was partially worn down, and the engine exhaust was worn consistent with sliding on a hard surface. The airplane was ferried to a facility near Montgomery, New York, where the mechanic examined the squat switch, the safe gear indication system, and the landing gear horn. He identified no anomalies with any of the systems. The mechanic added that he tested the squat switch by breaking into the wire bundle where it entered the fuselage. He did this to insure none of the wires had broken where the wire bundle flexed to accommodate landing gear retraction and extension. According to a Federal Aviation Administration inspector, a antenna on the bottom of the airplane pushed into the fuselage and damaged a former, requiring a major repair.