Three labourers died on Wednesday after inhaling toxic gases while cleaning a chamber of a sewage treatment plant in Mumbai’s Mira Road. One labourer is undergoing treatment at a hospital and his condition is critical, The Indian Express reported.

According to a statement issued by the Mira Bhayandar Municipal Corporation, which runs the treatment plant, the men entered the chamber to clean it without any safety gear. They fell unconscious after inhaling toxic gases.

“They were rushed to a hospital nearby where three of them were declared dead on arrival,” said Deputy Superintendent of Police (Mira Road Division) Shantaram Walvi. Mafizzul, 19, Muzaffar Mohlik, 24, and Rafiq Mandal, 50, died at the hospital, while Aftar Mulla, 49, is undergoing treatment.

A private firm named SPML Infra had been given a five-year contract for operational maintenance of the plant. SPML Infra had given another company, Tandon and Associates, a sub-contract to oversee daily operations, according to the civic body. It also said the two-metre-deep chamber did not require manual cleaning.

“Labourers entering the chamber to clean the interiors is not part of the standard operating procedure,” head of the Water Supply Department, Suresh Wakode, told The Indian Express. “We are not sure why they went into the chamber.”

Corporation Commissioner Balaji Khatgaokar and Mayor Dimple Mehta have ordered an inquiry, according to Mumbai Mirror. Inspector Vaibhav Shinghare of Kashimira police station said a case of accidental death has been registered and police are investigating if there was any negligence.