A massive fish kill affecting 40 miles of the Ogeechee River was caused by a bacterial disease induced by environmental stress, environmentalists said Thursday. However, questions are still unanswered about what caused the environmental stress. “The cause of the environmental stress is still unknown; therefore, EPD continues to advise citizens in Bryan, Bulloch, Chatham, Effingham and Screven counties not to swim in or consume fish from the Ogeechee River until further notice,” said Kevin Chambers, an EPD spokesman who issued a press release around 6:30 p.m. Thursday, three hours after government officials held a briefing regarding the matter.