On June 2, 2002, about 1327 eastern daylight time, a Boeing 757-232, N694DL, operated by Delta Airlines Inc., as flight 482, was not damaged while maneuvering near Fairfield, Illinois. The 2 certificated airline transport pilots, 108 passengers, and 3 flight attendants were not injured; however, 1 flight attendant was seriously injured, and 1 flight attendant sustained minor injuries. Instrument meteorological conditions prevailed and an instrument flight rules flight plan was filed for the flight that departed Denver International Airport (DEN), Denver Colorado, destined for Cincinnati/Northern Kentucky Airport (CVG), Covington, Kentucky. The scheduled passenger flight was conducted under 14 CFR Part 121. According to a flight safety investigator at Delta Airlines, the airplane crossed the JDUNN intersection at flight level 290 (FL 290). Air traffic control (ATC) then cleared the flight to FL 270. The first officer was the pilot flying, the autopilot was engaged, and the seatbelt sign was illuminated. The captain stated that during the descent, at 27,700 feet, the traffic collision avoidance system (TCAS) sounded "traffic, traffic, traffic," and displayed a target approximately 15 miles ahead and 700 feet below. No resolution advisory (RA) was received, but the flight was in instrument meteorological conditions and the flight crew was unable to visually identify the traffic. The captain then instructed the first officer to stop the descent. The first officer stated the he "quickly" leveled the airplane. Subsequently, a review of the TCAS and check with ATC revealed that the traffic was at FL 260, not FL 270. During the level off, a flight attendant fell and suffered a fractured leg. The airplane landed uneventfully at CVG, and the flight attendant was transported to a local hospital. A review of Delta Airline's 757/767 operations manual revealed that deviation from an ATC clearance was authorized for an RA only.