Four construction workers died after being trapped inside an under-construction sewer under a manhole near Kudghat on Thursday, officials of the Calcutta Municipal Corporation (CMC) said.

The officials suspect the sewer line had become deoxygenated.

Three colleagues of the victims were undergoing treatment at Baghajatin State General Hospital.

All of them were engaged in the construction of the sewer. An official of the CMC said the civic body was building a sewer network along with a drainage pumping station in the area — Purba Putiary — to improve the drainage capacity.

The sewer lines are being built as part of the Calcutta Environmental Improvement Investment Programme (CEIIP). Work was nearing completion and the manhole where the four workers were trapped is barely 25 feet from the drainage pumping station.

“We have already built the sewer network in the area. The final connecting line between the sewer lines and the drainage pumping station is being built now,” said an official.

“The manhole under which the men got trapped was the intersection point of all sewer lines. From here the sewage will go into the drainage pumping station through one sewer line, which is being built now.”

The police identified the four deceased as Sabir Hossain, 19, Mohammad Alamgir, 35, Jahangir Alam, 22, and Liyakat Ali, 20. All four were from Malda’s Purba Talsai, under Harishchandrapur police station.

Hossain and Alamgir were declared dead at SSKM Hospital. Alam and Ali were declared dead at Baghajatin State General Hospital.

“The sewer lines coming to the manhole had sewage inside. We had built walls on four sides under the manhole where the men were going down to construct the remaining sewer line that will take sewage to the drainage pumping station,” a CMC official said.

“One of the workers was trying to find out whether the sewage was leaking through the walls or if the walls had developed any crack. He slipped and fell. The bottom of the under-construction sewer line is nearly 16ft deep there. When he cried for help, three other workers went down. All of them got trapped and could not come up. Fire brigade and disaster management personnel of the police came and brought them out,” the official said.

The cause of the death of the four workers was still not clear. A police officer said the bodies had been sent for post-mortem. The police are questioning the contractor who had hired the four men for the work.

A professor of engineering at Jadavpur University said that it seemed the workers died because of lack of oxygen. “If sewage had leaked through the walls and filled the area when the men got trapped, then there is a possibility of the men inhaling methane and hydrogen sulphide that are generated in sewage channels. It could also be that the place where they got trapped did not have adequate oxygen because of lack of air circulation,” said the professor.

“Ideally, the presence of anaerobic gases should be measured before someone goes underground to work, especially in a sewage line. Also adequate air circulation must be ensured so that there is enough oxygen inside. There are available machines to take these protective measures.”