Five persons, including two security guards, died of suffocation at the Adani Wilmar edible oil plant in Neemuch on Thursday afternoon.
Identifying the five persons as Yuvraj Singh Chandravat, Rahul Singh, Deepak Yadav, Rajendra Singh and Ravi, Neemuch superintendent of police Rudolf Alvares said the victims drowned while they were cleaning a septic tank at the plant.
"We are lodging a case and a forensic team has been called. We are investigating the matter from all angles," he said.
Sources said that at first, two workers entered the tank and started suffocating. "When they called for help, three others entered it to rescue them.
But it was in vain - all the five died," said Shiv Pal Singh Yadav, thana incharge at Neemuch.
Senior police officers rushed to the spot soon after the incident occurred, but it was too late.
Local BJP MLA Dileep Singh Parihar related the incident to chief minister Shivraj Singh Chouhan, who assured a thorough probe into the matter.
Arun Kumar Singh, the manager of the plant, confirmed the incident but refused to elaborate further.
Meanwhile, agitated villagers assaulted tehsildar Gopal Soni because he was allegedly "siding with the company".