On July 15, 2004, at 2304 central daylight time, a McDonnell Douglas DC-9-82, N585AA, operated by American Airlines Inc., as flight 1114, encountered turbulence during descent through 17,000 feet mean sea level (MSL) about 1.6 nautical miles southwest of Sheridan, Illinois. Night visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 121 passenger flight was operating on an instrument rules flight plan. One flight attendant was seriously injured. The flight originated from Albuquerque International Sunport Airport, Albuquerque, New Mexico, at 1916 mountain daylight time, en route to Chicago O'Hare International Airport (ORD) Chicago, Illinois, where it landed without incident. The captain stated the airplane was in a clean configuration, speed brakes retracted, wing level, and approximately 3-5 degrees nose low when they encountered "moderate turbulence" with a "severe bump" within a cloud. The gross weight of the airplane was 102,000 pounds. The seat belt sign had been on for at least three minutes. According to the captain, weather radar displayed a faint green return from the cloud before the event. No lightening or precipitation was observed from the cloud. The first officer stated the radar showed some activity south of their course. The radar indicated a line of three or four well defined cells on the 80-mile range "painting green." They saw lightening "well" south of the airplane. Directly ahead, on the northern most side of the line, the radar displayed five to seven green dots. They were descending through 15,000 feet MSL in clear night conditions. They could see the buildup in front of them and estimated the top to be about 17,000 feet MSL. He asked the captain if he wanted to come left but he captain declined. The first officer asked if he was to sit the flight attendants down. The captain said he would take care of it and gave the 'Prepare for landing' over the public address. They entered the buildup at 280 knots and experienced sudden moderate to severe turbulence. The captain turned left and they were out of the turbulence in less than 10 seconds. A flight attendant sustained a broken ankle following the turbulence event. There was no damage to the airplane. Flight data recorder information provided by American Airlines Inc., recorded at N 41 degrees 30.7 minutes, W 088 degrees 42.35 minutes, maximum and minimum vertical accelerations of +2.25 g and -0.52 g. The Federal Aviation Administration and American Airlines Inc., were parties to the investigation.