Some carcasses were found grouped around water holes, while others appeared dead "falling flat on the face," according to Niall McCann, conservation director of the British charity National Park Rescue.
Live elephants seen nearby appeared to be physically weak, and one of them was circling, unable to change direction, observers said. The other species in the region do not seem to have been affected by what killed the elephants.
The Botswana government is testing samples of the dead elephants, but has not yet determined a cause of death.
The unusual number of elephant carcasses was first recorded in early May, said McCann.
"It's appalling - we need to know what's going on," he said, adding that he couldn't remember another time when so many elephants died of a mysterious cause.
"800 of them hang around like a magnet for criminals," he added.
McCann said there were a number of possibilities for what could have caused death, including an elephant-specific parasite - or even Covid-19.
"What I would like to point out is that it could be a public health crisis," he said.
Whatever the cause, McCann said it was important to get to the bottom of it because the loss of elephant lives was already "significant globally," he said.
