During the approach, the flight crew confirmed the landing gear was down and locked. The first officer landed the airplane on runway 35L in a slight left crosswind. When the airplane touched down, the landing gear warning horn sounded. The right main and nose landing gears retracted, and the airplane slid to a halt on the right side of the runway. According to the flight data recorder, the landing gear was extended and indicated down and locked (source data is the position of the nose landing gear). The nose landing gear became “unlocked” (i.e., unsafe) 2 seconds after the radio altitude reached 0 feet. Hydraulic pressure dropped to approximately 2,700 psi momentarily, then returned to normal (about 3,000 psi). The following items were subjected to various tests and no anomalies were noted: proximity switch electronics unit (PSEU), right main landing gear harness and proximity switches, landing gear selector valve, hydraulic pressure sensing valve, landing gear control assembly. Hydraulic fluid samples drawn from the filter pack, right main landing gear down lock assist actuator, and the return line for the landing gear selector valve were analyzed. No significant anomalies were noted. Partial systemic wiring tests were unremarkable. Probable Cause: uncommanded retraction of the nose and right main landing gears during landing roll for reasons undetermined.