On June 5, 2001, at 0858 central daylight time, a Boeing B757-200, N563UA, Flight 1597, operated by United Airlines (UAL) experienced turbulence while en route to O'Hare International Airport (ORD), Chicago, Illinois. The 14 CFR Part 121 passenger flight departed Fort Lauderdale Airport (FLL), Florida, at 0710 eastern daylight time with ORD as the destination airport. While en route the flight was diverting around thunderstorm activity. The pilots illuminated the seat belt sign due to the potential for turbulence. Around Kokomo, Indiana, the airplane encountered turbulence and subsequently one male passenger was seriously injured and one female passenger received minor injuries. Neither passenger was wearing their seat belt at the time of the incident. None of the other 143 passengers, 5 flight attendants, or 2 pilots were injured. The flight was on an instrument flight plan. A UAL safety representative reported, "The flight had descended from FL280 to FL240 in preparation for landing at ORD. The flight was deviating around thunderstorm activity 120NM southeast of ORD when turbulence was encountered between 1356 and 1358 UTC. Before starting the descent from FL280 to FL240, an announcement was made to the cabin that the FASTEN SEATBELT sign was on and that turbulence was expected during the descent." In the UAL Captain's written statement, he reported, "Before encountering turbulence the seat belt sign was turn[ed] on, and a P.A. announcement was made, specifying the possibility of turbulence during descent." In the UAL First Officer's written statement, he reported, "After we encountered the turbulence, we contacted the purser via the interphone. The purser informed us that there was one passenger in a lavatory with an injury to the head, and a second passenger also in a lavatory with a broken ankle." United Airlines was notified that the male passenger received a trimalleolar fracture to the left ankle. The male passenger, who received the serious injury, reported he had unfastened his seatbelt and proceeded to the lavatory at the rear of the airplane. He reported that the fasten seat belt sign was not illuminated when he left his seat, and that the flight deck had not issued any advisories while he was waiting for the lavatory. He reported that while he was in the lavatory, the airplane encountered turbulence that led to his ankle injury. The female passenger, who received a minor injury, reported she was in the lavatory when the airplane encountered turbulence. She reported that it was a freak accident and that all you could do was hold on.