On September 6, 1997, at 2040 central daylight time, a British Aerospace ATP, N855AW, operated by United Feeder Service as United Express Flight 5748 from Chicago, Illinois, to Peoria, Illinois, experienced severe turbulence during cruise flight at 10,000 feet near Peru, Illinois. One flight attendant received a dual compound fracture to her leg, a second flight attendant received minor injuries. The Captain, First Officer, and 39 passengers were not injured. The 14 CFR Part 121 scheduled domestic passenger flight was operating in visual meteorological conditions on an IFR flight plan. The flight departed the Chicago O'Hare International Airport at 2015 cdt. The Captain declared a medical emergency after encountering the turbulence and the flight landed at Peoria at 2054 cdt. The Captain and First Officer both stated that they were in cruise flight at 10,000 feet mean sea level (msl) when the accident occurred. The Captain reported it was night and the weather was "... primarily clear with a few scattered clouds and good visibility." Both the Captain and First Officer reported that the seat belt sign was off. The Captain reported, "The two jolts to the aircraft occurred as we 'topped' a small group of scattered clouds that had no vertical development." He reported that the first jolt was severe enough to disconnect the autopilot and took control of the airplane as the First Officer turned the seat belt sign on. They then requested and received a climb to 12,000 feet msl from air traffic control. He reported, "... as I initiated a climb I could not see any clouds at all. The few scattered clouds that we must have hit were behind us." They were then informed by Flight Attendant "B" that the "A" Flight Attendant was injured. The First Officer went to the back of the airplane to assess the situation. Upon returning to the cockpit he informed the Captain that the "A" Flight Attendant was badly injured. The Captain declared a medical emergency with air traffic control. The flight continued to Peoria where an uneventful landing occurred and the injured Flight Attendant was removed from the airplane by paramedics and transferred to the hospital. Flight Attendant "B" reported that both she and Flight Attendant "A" were cleaning up the galley when the turbulence was encountered. She reported that the turbulence took both of them by surprise and that they both hit the ceiling and fell to the floor when it was encountered. She continued to report, "As I was getting up, the second big bump came. I had my right hand on the handle of the cargo door. I hit my back against the baggage door. [Flight Attendant "A"] had nothing to hold onto; she it the ceiling again. Both times her right leg was folded under her body. By this time, I got up and held onto the handle above the cargo door. We were hit with a third bump. I held [Flight Attendant "A"] down with my left hand and held the handle with my right til it stopped." Several passengers in the airplane, one of which was a paramedic, provided assistance to Flight Attendant "A". Flight Attendant "B" informed the cockpit of the situation, assisted the passengers, cleaned up the cockpit, and assumed the duties of the injured flight attendant prior to landing. A review of weather data failed to reveal any SIGMETs or AIRMETs in effect for the area where the turbulence was encountered. The weather for Peoria (40 miles southwest of the accident location) at 1945 cst (50 minutes prior to the accident) was reported as 10 miles visibility with clear skies. The forecast weather for Peoria was for six miles visibility and scattered clouds at 25,000 feet. The Peoria weather at 2054 cst(14 minutes after the accident was reported as 8 miles visibility with clear skies.