The HS-125 corporate jet was en route at 31,000 feet, when the low pressure fuel filter warning light illuminated. The pilot diverted toward Columbia Regional Airport, MO (COU), but before reaching Columbia, both engines lost power. Subsequently, the aircraft crashed about 2 miles east of the airport. An investigation revealed the fuel was contaminated with water. The fuel filter de-icing system was operational, but a post-accident examination revealed the de-icing tank was empty. This was the second flight since the deice tank had been serviced. The manufacturer’s information indicated the supply tank should have provided about 3 to 4 hrs of filter de-icing operation. PROBABLE CAUSE: “Inadequate preflight by the pilot(s), water contamination of the fuel, low level of deice fluid (methanol) in the deice tank, and subsequent ice blockage of the fuel filters. Darkness was a related factor.”