Three sanitation workers died on Sunday while cleaning a 40-feet-deep tank at a sewage treatment plant in Uttar Pradesh’s Ghaziabad, The Times of India reported. The incident occurred in Dabur Talaab Colony in Loni area.

Deputy Superintendent of Police Durgesh Kumar said Bulbul, Mahesh and Roshan fell unconscious after inhaling poisonous gas inside the tank and died later, PTI reported.

The sewage treatment plant and the pumping station are owned by the Uttar Pradesh Jal Nigam, but maintained by a private agency Envirocon Engineering Company, according to The Times of India.

Sunil Sharma, a worker who was at the site, told the daily that Roshan had entered the tank by climbing down a ladder fixed on the tank’s wall. He had a rope tied around his waist. “He shouted he was having difficulty breathing,” Sharma said. “After a while, he untied the rope and started climbing the ladder. But soon, he started shivering and collapsed into the knee-deep sludge.”

Sharma said Mahesh climbed into the tank to help Roshan, but he too fell into the sludge. Bulbul also died while trying to save them.

Additional District Magistrate Sunil Kumar Singh said an inquiry has been ordered into the deaths. “The private agency has been named in the First Information Report on the basis of a complaint filed by the Jal Nigam,” Singh said.