Pesticides are believed to be behind nearly 100 deaths in Saharanpur and neighbouring districts in Uttar Pradesh over the past two months.
A team of Industrial Toxicology Research Centre scientists reached such a conclusion after examining people in the affected areas.
ITRC sources also ruled out the possibility of Japanese encephalitis being the cause of the deaths.
Earlier, experts from the National Institute of Communicable Diseases, Lucknow, had hinted at the possibility of mosquitoes being the carrier of the killer encephalitis virus.
Medical teams are in the process of taking blood samples of the victims to confirm their conclusion.
State Director General of Health Dr Gayatri Sharma said, "Organophosphate-based pesticides easily dissolve in water and the consumption of that water can cause convulsions and tremors often causing death."
"The symptoms of the victims were typical thereby making it all the more certain that pesticides alone were behind the deaths," she said.