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.

Zoo authorities in Hong Kong are keeping close watch on a monkey that is behaving unusually a day after the ninth death this week of one of the animals, while hastening tests to find out what killed them.

A zoo in Hong Kong is investigating the death of nine monkeys, including three members of a critically endangered species, in two days. Part of the Hong Kong Zoological and Botanical Gardens, the oldest in the territory, has been sealed off and disinfected, Hong Kong leader John Lee said in his weekly press briefing on Tuesday.