New Delhi, July 27 -- An 18-year-old daily-wage labourer and a 32-year-old civil contractor died of suffocation after allegedly inhaling toxic fumes inside a non-operational septic tank of an under-construction building in southwest Delhi's Qutub Vihar on Friday, police said on Saturday. The victims were identified as Pradeep Kumar and Subhash Mandal, both originally from Bihar. Police and the victims' families alleged that the two were not provided with any safety gear. A case of causing death by negligence has been registered against unidentified persons. Police said, the incident took place around 10.30am at a site in B-block of Qutub Vihar Phase 2, where the two were removing scaffolding from a 10-foot-deep and 9-foot-wide septic tank. "Although the tank was non-operational, it may have turned into a noxious gas chamber. It was sealed soon after construction, and its walls were waterproofed with chemicals a month ago. We are awaiting autopsy to confirm the cause of death," said a senior police officer. DCP (Dwarka) Ankit Singh said the Chhawla police station received a call about the incident from the hospital on Friday. Police said Mandal had taken up the work from Sujit Jha, a building material supplier, who, along with a person identified only as Tripathi, was supervising the five-story construction. "Pradeep went in first and began suffocating. Mandal entered to rescue him but also collapsed," said Chandra Shekhar Shah, Kumar's brother-in-law. "Exposure to toxic fumes is suspected," said DCP Singh....