"They were not our staff. However, we have ordered a probe to find out how these men entered the sewer without authorisation," a DJB official said.
Around 11.30 am, one of the men had gone inside the sewers to clean it. When he did not come out, a second worker went inside to check on him, police said.
Another man entered the sewer later to check up on the two, but he did not come out either following which the contractor, who had been assigned the cleaning job, came there and sent a fourth man in the sewer, they said.
Police arrived on the scene and pulled the three men out from the sewer. They were rushed to the AIIMS Trauma Centre where the doctors declared them "brought dead".
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Police said the three men died due to inhaling toxic fumes inside the sewer pipe.
Two of the deceased were identified as Joginder (32) and Annu (28), while the identity of the third victim was yet to be ascertained, police said.
The fourth worker, identified as Rajesh, had also inhaled the toxic fumes. He was admitted to a hospital where his condition is stated to be critical, they said.
"I request the LG for a probe into the matter and compensation for the families of the victims," he said.
"This is not the first incident of death of safai karamcharis (sanitation workers) during sewer cleaning and the attitude of Delhi Jal Board officials as well as the Arvind Kejriwal government continues to be callous," Tiwari said.
Last month, four sanitation workers died due to asphyxiation when they had entered a septic tank to clean it in south Delhi's Vasant Kunj. Two persons were arrested in connection with the deaths.