In May and June, conservationists discovered the carcasses of 356 elephants in Botswana's Okavango Panhandle. Some of the animals appeared to have collapsed and died suddenly while walking or running. Others seemed disoriented, walking in circles before they died.

While the elephant deaths have ceased, the cause has remained a mystery. None of the bodies contained bullet holes and no tusks were removed. Some experts suspected that poisoning might still be to blame, while others thought something in nature was the most likely explanation.

On Monday, Botswanan officials announced their answer: Neurotoxins produced by cyanobacteria, a type of microscopic algae, caused the elephants to die after they drank from large puddles that formed after rains. While some conservationists accepted the explanation, others did not consider the mystery solved and feared that the elephants could again face renewed danger.

Cyril Taolo, deputy director of Botswana's Department of Wildlife and National Parks, said, "There's absolutely no reason to believe that there was human involvement in these mortalities. This is … something that happens quite a lot when there are these environmental changes."

Mmadi Reuben, the government's principal veterinary officer, added that "there are a lot of questions that still need to be answered," including why elephants were the only species that died and what caused the outbreak.

"There's just so many questions that are outstanding," said Pieter Kat, director of LionAid, a nonprofit conservation organization. Kat has worked extensively in Botswana and researched wildlife diseases there. "They need to be completely transparent about the laboratories the samples were sent to and the lab reports."

Most types of cyanobacteria that produce neurotoxins occur in marine environments, where they can cause red tides. A few species are found in freshwater around the world, and they sometimes kill dogs, cattle and other animals that drink from or swim in contaminated water bodies.

"One of the biggest unresolved issues is why there seems not to have been collateral mortality," said Chris Thouless, head of research at Save the Elephants, an organization based in Kenya. "That is one of the reasons we originally said this was not a probable explanation, because other animals didn't seem to be dying."

Elephants account for the majority of wildlife in the area, but cattle are also found there, Thouless said. He has not been able to verify, however, whether cattle drink from the same bodies of water used by elephants.

There are several possible explanations for why only elephants died, said Roy Bengis, a veterinary wildlife specialist at the University of Pretoria in South Africa, who formerly served as chief state veterinarian at Kruger National Park.

It could be that elephants are "exquisitely sensitive" to whichever particular neurotoxin killed them, whereas other species are more resistant, Bengis said. "We know this happens — different species of animals have different tolerances."

Elephants also drink copious amounts of water, up to 40 gallons a day, so they would be taking in a larger dose of toxin than a smaller animal. Additionally, unlike most other species, elephants "actually go and frolic in the water and roll in the mud and spray themselves," Bengis said. The neurotoxin might have been absorbed through their skin.

Why vultures and other scavengers were not affected is another unanswered question. A neurotoxin would most likely have been concentrated in the elephants' relatively small and inaccessible brains and spinal cords, making it less likely to be consumed, Bengis said.

But not all conservationists are convinced that toxic algae are responsible. "The chance was lost to find out what has really happened to the elephants, because of the government of Botswana's unwillingness to collaborate with the research community at an early stage," said Keith Lindsay, a conservation biologist.

Bengis added that without seeing the lab results or knowing more about the samples, "I can't say yea or nay with regards to whether this diagnosis is correct or incorrect."