Nine monkeys who died in Hong Kong’s oldest zoo in two days this week had been infected with an endemic disease, possibly after some digging work near their cages, officials said on Friday.

The nine monkeys which died at the Hong Kong Zoological and Botanical Gardens have been found to have contracted melioidosis, the government has said. The disease can infect both humans and animals, though the incident is believed to be an isolated infection within the zoo.

Hong Kong’s oldest zoo is seeking answers in a monkey medical mystery after nine animals died in two days, including three members of a critically endangered species.

The animals contracted melioidosis, possibly after some digging work near their cages, that later caused them to develop sepsis, officials said.