On June 20, 1995, about 1810 central daylight time, a Boeing 767- 222, N600UA, operated by United Airlines as United Airlines Flight 9, experienced severe air turbulence approximately ten miles south of Champagne, Illinois, and lost approximately 900 feet of altitude. The flight crew diverted the flight and made a precautionary landing at Lambert-St. Louis International Airport, St. Louis, Missouri. The airplane sustained no damage. Three flight attendants, of a crew of 11, reported serious injuries. One of 137 passengers, who had been seated but had not had her seat belt fastened, reported serious injuries. The scheduled domestic passenger flight operated under 14 CFR Part 121. The flight originated from Kennedy International Airport, New York, New York, at 1625 eastern daylight time with a planned destination of the San Francisco International Airport, San Francisco, California. An IFR flight plan was filed, and instrument meteorological conditions (IMC)existed along the route of flight. According to the captain's written statement, he was deviating the flight from its planned course line to avoid thunderstorm activity. The captain stated he observed on radar an opening between two cells of approximately 45 miles. He stated that a route between these cells would allow their flight to return back to its course. While vectoring between these two cells in IMC, the airplane momentarily entered into visual meteorological conditions (VMC) and the captain and first officer saw a cumulus nimbus buildup directly ahead of their path. The captain stated that this buildup had not been displayed on their radar despite use of various ranges and tilts. He stated that the buildup was too close to avoid. The captain immediately made an announcement over the PA for the flight attendants, who were in the process of serving the cabin, to be seated. But he stated that there was not enough time for the flight attendants to react before the airplane penetrated the top of the cumulus buildup. The captain reported they encountered ten seconds of severe turbulence and lost approximately 900 feet of altitude. The airplane was level at flight level 390 approximately 120 miles northeast of St. Louis, at the time of the turbulence encounter. The flight diverted to St. Louis International Airport for medical care for the injured flight attendants and passenger. The airplane landed without further incident. The first officer described the flight up until the turbulence encounter as smooth to light chop. He stated that the seat belt sign for the passengers had been turned on for approximately eight to ten minutes prior to cell penetration. The nine flight attendants described the flight up until the encounter as normal and smooth. Two of the nine flight attendants also mentioned light chop. During a telephone interview, the first flight attendant stated that the captain did give a preflight briefing and it was normal. She did not recall anything mentioned out of the ordinary, nor any mention of bad weather. Convective Sigmet 39C was issued at 2255 Zulu and was in effect for an area that included the approximate location of the turbulence encounter. Convective Sigmet 39C indicated an area of severe thunderstorms moving from 040 degrees at 15 knots with tops above 45,000 feet. It also indicated hail to one inch and possible wind gusts to 50 knots. The Enroute Terminal Forecast for Indianapolis at 1800 central daylight time indicated a chance of four miles visibility, thunderstorms, and light rain showers. The captain was not able to recall Convective Sigmet 39C being broadcasted. The first officer stated that it had not been broadcasted. An inquiry was made to the FAA concerning the time in which 39C was broadcasted. The FAA indicated that a transcript was prepared which covered the time frame of five minutes before the incident until five minutes after the incident. The FAA stated that in reviewing the transcript, it was not possible to determine when 39C was broadcasted from the existing data.