An Air Wisconsin Airline flight, operating as United Express Flight 684, had been cleared to descend from FL190 to 13,000 feet on the Thompson One Arrival into Denver. As the airplane was passing 14,000 feet, it encountered unreported and unanticipated clear air turbulence. Two flight attendants, making their pre-landing check of the cabin, were seriously injured. One flight attendant received fractures to her pelvis, collarbone, and two ribs. The other flight attendant sustained a fractured ankle.