Senior district officials and members of the pollution department are probing the mysterious death of thousands of fish in Gorakhpur's Rapti River.
Officials have warned villagers not to take and eat fish as the exact cause of the death is still unknown.
There was a mad rush of villagers to catch fish once the news spread.
A senior district official, Ranjan Kumar, said they were probing the mysterious death of the fish.
Kumar said it was suspected that the pollutants dumped into the river by a sugar mill located on India-Nepal border could be the reason behind the incident.
"It looks like the waste coming from a sugar mill and distillery at the border might have caused oxygen levels to drop in the water. A team from Pollution Board has come to analyse the water; even they suspect that it could be because of the pollutants. They will test and get to know the oxygen level in the water. A research team from Fishery department from Lucknow will also be here for inspection," said Kumar.
Dr. Ranjana Tripathi of the pollution department said that the initial findings pointed towards toxicity in the river water.
"Not all the fish are dead; some are alive but floating on the surface. It looks like a case of toxicity, exact reason can be determined only after tests are completed," she said.
Local municipal authorities have also visited the spot and are in the process of cleaning the river with some chemicals.