Traffic police in Hebei province discovered 18 macaques hidden in the luggage compartment of a bus on Tuesday afternoon, three of them already dead from suffocation.

Highway police near Handan found four bamboo baskets containing the rare monkeys during a routine vehicle inspection. The presence of the monkeys was apparent to the police officers due to a strong smell and noises being made by the animals, according to a report in Yanzhao Evening News.

The animals, which were of a rare and officially protected species, had been transported illegally from Yunnan province with nothing to eat or drink for two days.

They were sent to a zoo in the city for medical treatment on Wednesday morning.

However, an expert at the zoo said that the 15 monkeys that survived had been under severe strain and might still not survive their ordeal. They range in age from 1 to 6 years old.

The bus driver and passengers said they did not notice the animals on the bus.

The bus owner said he did not have any information on the goods or luggage carried on the bus or who would be receiving it. His sole responsibility, he said, was to deliver the bus and its contents to their destination.

Police from Hebei and Yunnan provinces are investigating the case.