Police launched an appeal to the general public on Tuesday for assistance in tracking down five squirrel monkeys from a small nature park in Kiryat Motzkin.
The primates played a key role in a program for children with Down's Syndrome, police added.
A sixth monkey has already been retrieved by Israel Nature and Parks Authority inspectors during a raid on the home of a suspect.
The monkeys were stolen by an unknown suspect who would feed them snacks laced with sleeping drugs. After the animals fell asleep, the suspect would place the monkey in a bag and leave the area.