Melioidosis, a bacterial infection, was responsible for killing at least nine monkeys at a Hong Kong zoo, authorities said, as a further two died over the weekend, taking the total to 11 in the past week.

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.

A 11th monkey from the Hong Kong Zoological and Botanical Gardens was found dead on Sunday whilst under quarantine, the government has said. In a statement issued on Sunday evening, the government said a common squirrel monkey died while being placed under isolated surveillance.

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.

Common squirrel monkey was one of two that was put under isolated surveillance last week after showing signs of illness.