Two workers died and two others fell ill while working inside a septic tank being built in Nathuapara on the outskirts of Jalpaiguri town on Tuesday afternoon.

The deceased have been identified as Alim Mohammad, 31, and Ananta Roy, 24. Two others, Rahim Mohammad and Bipul Roy, both in their mid-30s, are under treatment at the superspecialty hospital in Jalpaiguri.

Sources said Narayan Roy, a resident of Nathuapara, was making a septic tank at his home. A few days back, mortar was laid on the top of the tank. On Tuesday, Alim Mohammad got inside to take out the wooden planks put for support, but slumped senseless. Ananta got down to help him, but he also fainted. Rahim and Bipul also got in, but when they felt uncomfortable, they called for help and some bystanders helped them out.

As the other two lay unconscious inside, residents informed the Jalpaiguri fire station. Firemen brought Alim and Ananta out, but doctors at the Jalpaiguri superspecialty hospital pronounced them brought dead.

“We suspect they have died of oxygen shortage in the septic tank (that was closed),” said an officer of Kotwali police station.

House owner Narayan said the tank was under construction and hence without garbage, so there was no possibility of noxious fumes like ammonia. He added that the tank was 9 feet long and 7 feet deep.

Doctors at the hospital said Rahim and Bipul were being treated. “We are waiting for post-mortem reports to know the cause of deaths of the other two workers,” said hospital superintendent Gayaram Naskar.

The incident has shocked residents of the area. “We are perplexed at the deaths,” said resident Lutfar Rehman.