Officials at Sao Paulo zoo say the death toll in a wave of mysterious animal poisoning has risen above 50.
Three chimps are among the nearly 60 animals dead so far
Laboratory tests showed that the animals, who all died from multiple organ failure, were given a rat poison banned in Brazil.
A state official said accidental death had been ruled out because of the high dose found in the animals.
Victims over the past week include monkeys, golden-headed lion tamarinds and more than 30 porcupines.
Three chimpanzees, an orang-utan, three tapirs, four camels, an elephant and a bison died during the previous month.
Search for the suspected killer or killers has intensified and police are hopeful that they will track them down.
Police inspector Clovis Ferreira de Araujo said the person involved had to know the zoo's routine and had a background in chemistry.
Interviews have been carried out with current and former employees as well as directors of the zoo, officials of the companies working with the zoo and vets.
The zoo's administration has ordered all its staff to work in pairs and has increased security by hiring more security personnel and installing closed-circuit cameras.
"If it's someone from inside of the zoo, I can't understand
what is going through this person's mind," said a security
guard quoted by Reuters news agency.
"If this is an outside job, I can't understand how they are
managing to do it, given our strict security measures."
The killings have attracted international attention.
A spokeswoman for the American Zoo and Aquarium Association said there was cause for concern for all animal lovers and that it was shocking anyone would want to do such a thing.