Fourteen adult elephants were found deceased by villagers on Thursday, and another four bodies were found dispersed on the foothills of the Kundoli reserve forest in Nagaon district.
Locals who found the elephants said the animals could have been killed by a lightning strike on Wednesday.
A local forest ranger said he had seen burnt trees in the area.
The carcasses have been sent for post-mortem examinations, and the state government has ordered an inquiry into the incident, Assam’s forest and wildlife minister Parimal Suklabaidya said.
"A preliminary report suggests the deaths could be due to lightning although we need to find out through forensic tests if there could be any other reason like poisoning or disease," he said.
"This is a very sad incident, such incident has never occurred in the forests of Assam.”
Despite claims that lightning may have been the cause of the elephants’ death, prominent conservationist Soumyadeep Datta said that was unlikely, based on images shared on social media.
“Poisoning could be behind the death of the elephants,” Datta said. “We have to wait for the autopsy report, which the forest department will do soon.”
This is the first time in 20 years in Assam that so many elephants have been found dead at the same time.
India is home to nearly 30,000 elephants, about 60 per cent of the wild Asiatic elephant population.
An estimated 6,000 or more wild Asiatic elephants live in Assam. They often come out of the forests in search of food.
However, their population has dwindled due to poaching and habitat loss.
Conservationists have urged the government to limit people’s encroachment into elephant territory and to establish free movement for the elephants to travel between forests safely.
In recent years, wild elephants have entered villages, destroyed crops and even killed people.