Four persons died Sunday allegedly of asphyxiation after they entered a sewage treatment plant in west Delhi's Moti Nagar area, police said.
The incident took place at 3:30 pm, they said.
The victims were rushed to a hospital where they were declared brought dead by the doctor, a senior police officer said.
The dead were identified as Sarfaraz, Pankaj, Raja and Umesh, he said.
Meanwhile, another victim Vishal, who also accompanied them to the plant is critical and is being treated at the Ram Manohar Lohia Hospital, the officer added.
The victims died allegedly after they inhaled toxic gases, police said.
A case has been registered, police said, adding that they are probing whose negligence was it that triggered the incident.
Further investigation is underway, they said.