From the Aviation Safety Network's crash Report
Historical, Airplanes - Wreckage
The Co-Pilot was the flying pilot with the Captain giving directions throughout the approach phase. The Captain stated that he extended the flaps and the landing gear. When the aircraft touched down, the landing gear was not extended.
PROBABLE CAUSE: "The landing gear down and locked was not verified prior to landing. The checklist was not followed, and an inoperative landing gear warning horn were factors."