The carcasses were found scattered in the forests yesterday, officials said.
"Local forest staff recovered nine carcasses on Sep 4 and the rest were found yesterday," Divisional Forest Officer (DFO), South Kheri, Sameer Verma said.
"A postmortem was carried out by veterinarians. No foul play has been found," he said.
Though the postmortem report is yet to be released, officials suspect that the simians may have died due to heart failure.
Wildlife conservationist Vijay Prakash Singh, who is also convenor of Terai Nature Conservation Society (TNCS), said pesticides could also be a reason behind the sudden death of monkeys.