While cleaning the 25-ft deep chamber, Halka Lal (25) slipped from a step inside on which he was standing and fell down. Kalla Jatav (22), another sanitary worker, too fell in while trying to help him, inspector Sanjay Mishra of Kotwali police station said.
Both the labourers, hailing from Gwalior district and hired by a private contractor, died due to suffocation before they could be rescued, Mishra said.
"We have registered a case and further investigations are on," the inspector said.