Hundreds of mysterious elephant deaths in Botswana were probably caused by a naturally occurring toxin, a senior wildlife official told Reuters.
Cyril Taolo, the acting director of the Department of Wildlife and National Parks, said preliminary tests show it's highly unlikely an infectious disease killed at least 281 elephants.
Officials had been struggling to establish the cause of death more than two months after the first carcasses were spotted in the Okavango Panhandle region.
Initial investigations appeared to rule out common causes like poaching and anthrax.
The government has sent samples to laboratories in Botswana, South Africa, Zimbabwe and the United States for tests.
The elephant deaths have concerned some conservationists, who fear deaths could spiral out of control if a cause cannot be established quickly.
Africa's overall elephant population is declining due to poaching but Botswana, home to almost a third of the continent's elephants, has seen numbers grow to around 130,000.