Authorities in Botswana say that toxic cyanobacteria, a kind of algae found in water sources, is behind the mysterious death of elephant herds earlier this year.

Dr. Mmadi Reuben, the principal veterinary officer for the Department of Wildlife & National Parks, said Monday that the laboratory samples of water obtained from elephant drinking sites in and around Seronga, in the northwest part of the country, were positive for the toxic algae.

“We observed a lot of elephants who were dying,” Reuben explained as he announced the findings on Botswana state television. The deaths of hundreds of elephants, which drew international attention, were clustered around the region’s water sites but did not appear to affect vultures, hyenas and other scavenger species.

The samples analyzed in labs in Botswana and Zimbabwe were negative for a range of infections that might have affected the elephants. They also were negative for toxins including heavy metals and pesticides. But the samples were positive for cyanobacteria and their detection led to the diagnosis.

“A lot of questions still need to be answered,” said Reuben. Among them are why the elephants were affected but not other species, and why the cyanobacteria present in the Seronga locations proved so lethal.

The government will continue to monitor the elephant populations, he added.