Carcasses of the monkeys, aged between one-and-half to 10 years, were exhumed from a rubber plantation owned by a villager at Senanipuram on Sunday.
Forest officials carried out a search in the area based on information that a number of monkeys had been killed and the carcasses dumped en masse in a pit.
"The informers told us that they had seen the carcasses of 15 monkeys. But, we could find carcasses of only seven monkeys as of now. Prima facie, the animals were fed with food mixed with a pesticide," forest range officer Abdul Jaleel said.
Forest and police personnel have zeroed in on a person as the prime suspect behind the crime and he would be tracked and arrested soon.
"Wild animals especially monkeys used to stray into the area and destroy crops and trouble the people. The culprit might have done this cruelty to end the menace," the official said.
The carcasses have been sent to the Veterninary Biological Institute at Palode for chemical analysis and postmortem, the official added.
Monkeys are included under Schedule 3 of the Wildlife Act and their poachers are liable to get three years rigorous imprisonment and a fine of Rs 25,000.