On June 5, 2005, around 1300 central daylight time (cdt), a Boeing 757-223, N602AN, operated by American Airlines as Flight 1399, encountered moderate turbulence while descending through 13,000 feet mean sea level (msl) near BEARZ intersection, New Chicago, Indiana. Instrument meteorological conditions prevailed at altitude. The commercial flight was operating under the provisions of 14 Code of Federal Regulations (CFR) Part 121. One flight attendant was seriously injured and another flight attendant sustained minor injuries. The captain, first officer, 2 remaining flight attendants, and 156 passengers were not injured. The flight departed Miami International Airport (MIA) at 1124 eastern daylight time (edt) and arrived at Chicago O'Hare International Airport (ORD) at 1320 cdt. The captain reported that as the airplane approached the BEARZ intersection he noticed a developing line of clouds with cloud tops ranging between 15,000 and 18,000 feet msl. The captain stated that he believed that the remainder of the flight "might be bumpy, but not excessively so, because of a lack of bad ride reports from other aircraft passing through the same area and the low altitude of the clouds." The captain reported that five to eight minutes before encountering the turbulence he made a public announcement and told the flight attendants to prepare for landing. The captain stated that the airplane encountered "moderate turbulence" while descending in the clouds. The captain reported that after clearing the clouds the "ride smoothed out" and he called the flight attendants and was told that two flight attendants were injured and were being treated by physicians. The captain requested that paramedics meet the airplane after landing. The flight attendant who was seriously injured was located in the aft galley preparing for landing when the airplane encountered the turbulence. The flight attendant stated that she heard the captain say "prepare for landing" and "all of [a] sudden I got lifted off the ground and slammed into the 4R door." This flight attendant suffered a fractured ankle. The flight attendant who sustained minor injures was located near the aft lavatories preparing for landing when the airplane encountered the turbulence. The flight attendant stated that the airplane "hit some type of severe turbulence without warning" and that she was "thrown to the floor." This flight attendant suffered minor injuries to her shoulder, hip, foot, and lower back. She elected to complete the remainder of her scheduled flights. Data from the airplane's Digital Flight Data Recorder (DFDR) was extracted and indicated that the airplane experienced vertical accelerations that were consistent with a convectively induced turbulence (CIT) event. The CIT encounter was recorded at 1259:42 (hhmm:ss) and lasted about 4 seconds. During the CIT encounter, the maximum and minimum vertical accelerations were about 1.9g and 0.5g, respectively.