On March 2, 1997, at 0555 central standard time, a Boeing 757-2Q8, N755AT, operating as Mexicana Airlines Flight 199, experienced turbulence approximately 120 miles south of Chicago, Illinois. The international 14 CFR Part 129 flight was descending through 26,000 feet. The flight departed Durango, Mexico, at 0335 with the intended destination of Chicago, Illinois. The airplane sustained no damage; however, of the 202 persons aboard, 18 suffered minor injuries and 4 sustained serious injuries. Visual meteorological conditions prevailed at the destination at the time of the accident. The Captain stated in his written statement that the turbulence began when the airplane descended through 26,000 feet and continued until 20,000 feet. He said that when passing through 26,000 he made an announcement to the passengers to take their seats. He said that during the turbulence he told passengers to fasten their seatbelts. He said that after the turbulence encounter the "Fasten Seatbelt" sign remained on. The First Officer, who was at the controls at the time of the turbulence encounter, said in his written report that descending through 28,000 feet he felt a little turbulence and discussed reducing penetration speed with the Captain, who agreed. He said that from 26,000 to 20,000 feet the airplane encountered severe turbulence with rate of climb reaching a 5,000 foot per minute descent and climb respectively. He said the auto pilot was turned off, and the turbulence lasted about two minutes. He said that while this was happening he told the Captain to turn on the "Fasten Seatbelt" light. He indicated that after the turbulence encounter the Captain took control of the airplane. A flight attendant described the turbulence encounter in her written statement stating that when it started she made an announcement for passengers to return to their seats and fasten their seatbelts. This announcement was given in English and Spanish. She indicated that the two most seriously injured passengers were in the lavatory when the turbulence encounter occurred. The company debriefed the crew. The company stated that the encounter was in clear air and they characterized it as "clear air turbulence." The company reported that the pilots received no previous warning of turbulence while in-flight. They did say that the Captain obtained meteorological information in Durango and talked with an arriving Captain who briefed him on the route conditions stating that he had encountered light to moderate occasional turbulence. The accident occurred on Sunday (March 2nd). Notification was received by the NTSB at 1008 on March 3rd. Prior to receiving notification, the airplane had been inspected by company maintenance personnel while in Chicago on the date of the incident and no external damage was found. The airplane was then returned to service and had made a return trip to Mexico the afternoon of the same day. There was no attempt to isolate the cockpit voice recorder or the digital flight data recorder prior to further operation of the airplane.