Over 350 elephants have died in the southern African country of Botswana since May, leaving conservationists puzzled, BBC reported on Wednesday. Botswana is home to one-third of the continent’s elephant population.

An expert from a United Kingdom-based charity called National Park Rescue told BBC that his colleagues in Botswana had first spotted elephant carcasses in the Okavango Delta region in May, while they were on a flight. “They spotted 169 in a three-hour flight,” Dr Niall McCann told the broadcaster. “To be able to see and count that many in a three-hour flight was extraordinary.”

McCann added that further investigations revealed many more elephant deaths, taking the total to over 350.

The Botswana government was alerted about the elephant carcasses in May. They had ruled out poaching as a cause of the deaths, saying the tusks had not been removed. McCann also pointed out that other animals would have died if the poachers used cyanide.

McCann ruled out poisoning but said that the way the animals were dropping dead and their behaviour before dying indicated a neurological condition. The elephants were seen dropping to their faces while others were seen walking in circles.

An official from Botswana’s Department of Wildlife and National parks told The Guardian that 280 elephant deaths have been confirmed by the government. “We have sent [samples] off for testing and we are expecting the results over the next couple of weeks or so,” he told the newspaper. “The Covid-19 restrictions have not helped in the transportation of samples in the region and around the world. We’re now beginning to emerge from that and that is why we are now in a position to send the samples to other laboratories.”