The flight climbed to 33,000 feet after departure, where it encountered turbulence. The captain turned on the seat belt sign and announcements were made to the passengers to remain seated with their seat belts fastened. ATC informed the pilots that other pilots reported smoother air at 27,000 and 35,000 feet. They descended to 27,000 feet, encountered turbulence and then requested 35,000 feet. After reaching 35,000 feet, they encountered greater turbulence and requested and received clearance to 33,000 feet. During descent, they encountered severe turbulence which resulted in injuries to passengers. A SIGMET had been issued by the National Weather Service for active intensifying thunderstorms at the point of the turbulence encounter. The weather information supplied to the pilots by the operator did not contain this SIGMET; however, it did contain a company forecast for possible isolated thunderstorms within showers as indicated in the flight release. Probable Cause: the turbulence and failure of the passengers to have their seat belts fastened. A factor relating to this accident was: the failure of the aircraft operator to advise the flight crew of forecast active intensifying thunderstorm conditions in the area of the accident.