On February 17, 2014, about 1254 mountain standard time (MST), United Airlines flight 1676, a Boeing 737-724, N23708, encountered severe turbulence about flight level 340 near Kaycee, Wyoming. Of the 120 passengers and crew onboard, two flight attendants were seriously injured, and one FA and nine passengers received minor injuries. The airplane was not damaged. The flight was operating under the provisions of 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from Denver International Airport (DEN), Denver, Colorado, to Billings Logan International Airport (BIL), Billings, Montana. The flight departed DEN normally and climbed to a cruising altitude of flight level (FL) 380. The flight encountered light turbulence and the crew requested, and was granted, a descent to FL360 followed by a descent to FL340. The flight crew extinguished the seat belt sign when the ride smoothed out. Flight data recorder (FDR) data indicated the airplane was cruising about FL340 when it encountered an area of severe turbulence for about 55 seconds. The most severe turbulence occurred for about 10 seconds when the vertical acceleration fluctuated between about -1.23 g's and +2.05 g's, the lateral acceleration fluctuated between about -0.27 g's and +0.23 g's, the pitch fluctuated between about +5.8° (nose up) and -1.8° (nose down), and the roll fluctuated between about 14° left and 20° right. At the time of the turbulence encounter, the two flight attendants (FA) in the forward galley and the one FA in the aft galley were injured. during the turbulence encounter and suffered. The aft FA was unconscious after the event and tended to by medical personnel passengers until landing. After being notified of the serious injuries, the flight crew declared an emergency with air traffic control and arranged for medical personnel to meet the aircraft at BIL. The three FAs were transported to the hospital. One FA was diagnosed with a fractured fibula; one was diagnosed with fractures of her scapula, a rib, cervical vertebrae, and thoracic vertebrae; and one was treated and released with minor injuries.