Hong Kong authorities are carrying out tests to find out what killed eight monkeys, which were found dead in the city’s oldest zoo on Sunday, the government said on Monday.

The animals, a De Brazza’s monkey, one common squirrel monkey, three cotton-top tamarins and three white-faced sakis, were found dead at the city’s Zoological and Botanical Gardens (HKZBG) on Sunday, Hong Kong’s Leisure and Cultural Services Department said in a statement.

While awaiting test results, the mammals section of the zoo will be shut from Monday for disinfection and cleaning.

“We will also closely monitor the health conditions of other animals. During this period, other facilities of the HKZBG will remain open,” the statement said.

The HKZBG is the oldest park in the territory. Built in 1860, it houses around 158 birds, 93 mammals and 21 reptiles in about 40 enclosures.