Delta Air Lines flight 278, a Boeing 767-300ER, encountered severe turbulence during cruise. Of the 95 passengers and crew onboard, one flight attendant received serious injuries and two flight attendants received minor injuries. The airplane was not damaged. The first officer was the pilot flying, and the captain was pilot monitoring. According to the captain, the flight had experienced continuous light turbulence during the climb. Upon reaching FL350, in the moonless night, and in smooth air, the captain said that he deactivated the seat belt sign, and released the flight attendants to begin cabin service, while also giving a precautionary notice on the interphone to remain seated with the seatbelts fastened as much as possible. He added that the weather radar was set with the Gain on AUTO, and it was depicting green radar returns on its display. He said that they were approaching SUNNS intersection and noted that the radar showed a light green return. He said he discussed it with the first officer, and they both agreed to request a turn and a climb to avoid the depicted weather. After clearance was given to make the turn, the captain said that he turned the seatbelt sign ON, and made an announcement for everyone, including the flight attendants, to be seated. As the flight made the course deviation to avoid the weather, the captain indicated that the radar gave more definition in its depiction, and the lightning illumination emanating from the cloud showed that the cloud extended about 1,000 feet above their flight level, so they continued the turn to remain outside the green weather return depiction. He said that the radar depiction suddenly showed a faint dot of red at its center, and at that time the ride went immediately from smooth to severe turbulence, without any intervening intermediate intensities being noted. He said the airplane climbed about a few hundred feet and a momentary engine indicating and crew alerting system (EICAS) overspeed message occurred. He further stated that the turbulence came in two waves, and it lasted less than 30 seconds. The airplane then recovered back to FL350, and light chop continued. During the turbulence encounter, the flight attendants were in the middle of meal service. As a result, two flight attendants sustained minor injuries and one flight attendant fractured an ankle. Probable Cause and Findings The airplane?s encounter with severe turbulence.