Three contract workers died in a mishap at a unit of the Haldia Petrochemicals Ltd (HPL) in East Midnapore district of West Bengal.
Describing the deaths as an "unfortunate accident", the HPL authorities said the workers had gone inside the process chamber last midnight.
A spokesman of HPL said detailed inquiry was in progress and initial findings indicated that the persons were employed by a contractor firm.
The authorities said contrary to earlier reports there was no gas emission in the plant. The plant had been shut down since May eight for maintenance, the spokesman said.
District Magistrate S D Chhoten Lama said the bodies were later retrieved from the chamber and sent for post-mortem.
She said initially it appeared that the three had died of suffocation. However, the HPL authorities were inquiring into the exact circumstances of the deaths, she added.
HPL authorities said all support would be extended to the bereaved families.