Researchers hoped to use an aerial survey to count elephants in Botswana. Instead, the survey discovered one of the largest elephant slaughters to date.

Eighty-seven elephants lay dead, most of them with their skulls chopped off near a wildlife sanctuary in the Okavango Delta. Poachers tried to hide the carcasses beneath drying bushes, NPR reported Monday (Sept. 3). [Elephant Images: The Biggest Beasts on Land]

Mike Chase, the director and founder of Elephants Without Borders, the nonprofit that conducted the survey, told the BBC that the poaching incident is one of the largest he's seen or read about in Africa. In the same area, three white rhinos were poached and killed in the past three months, according to NPR.

Ivory poaching is a huge problem for the continent; a third of Africa's elephants were killed in the past decade, according to the BBC.

But until recently, Botswana was largely unaffected, thanks to strong anti-poaching policies and armed anti-poaching units, according to the BBC. Indeed, tracking collars reveal that elephants that migrate from neighboring countries have largely stayed in Botswana.

This latest incident comes a few months after Botswana disarmed its anti-poaching units following the election of Mokgweetsi Masisi as president, according to the BBC.

There could be even more cases of poached elephants, according to the BBC. The aerial survey is only about halfway complete.

Originally published on Live Science.