Hong Kong's oldest zoo sealed off part of its grounds on Tuesday as it brought in experts to seek answers for the death of nine monkeys in just two days.
Three of the nine monkeys were of endangered species.
Authorities of the Hong Kong Zoological and Botanical Gardens are now keeping a close watch on a monkey which is displaying unusual movement and appetite.
The sealed parts of the zoo are being cleaned and disinfected.
In his weekly press briefing on Tuesday, Hong Kong leader John Lee said that experts had been called in to conduct necropsies and toxicological tests.
"Government departments will speed up autopsy and toxicological testing," the government said in a statement. "The health conditions of all 80 animals in the HKZBG are normal."
Meanwhile, officials held an urgent meeting on Tuesday.
What do we know about the monkeys?
Eight of the monkeys initially died on Monday as zoo authorities scrambled to pinpoint the cause of death.
They included a De Brazza's monkey, a common squirrel monkey, three cotton-top tamarins and three white-faced sakis.
The cotton-top tamarin species is listed as critically endangered by the International Union for Conservation of Nature.
The deaths raised concerns about a possible outbreak of a zoonotic disease such as mpox, according to Jason Baker, senior vice president of animal rights group PETA Asia.
"Monkeys in captivity are often exposed to pathogens that cause diseases that can be transmitted to humans, including tuberculosis, Chagas disease, cholera and MRSA," he said in a statement.