The constable, Anand Khatri (32), was posted in the southwest cell of the police's anti-terrorist unit around three months ago. During the operation, the bullet hit his chin, said a police officer.
"It was a case of accidental firing. The official was rushed to a hospital but his life could not be saved," Special Commissioner of Police (Special Cell) Arvind Deep said.
Sonu (30), who is allegedly associated with southwest Delhi-based gangster Manjit Mahal and history-sheeter Ravinder Bholu, was wanted in connection with nine criminal cases - including murder, extortion and robberies - across Delhi-NCR and Delhi Police had announced an reward of Rs 20,000 on his arrest.
Sonu was being tracked for around the past two months by a team led by two inspectors TIlak Chand Bisht and Rajesh under the supervision of ACP (Special Cell) Sandeep Byala.
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Once the team reached the single-storey house in which Sonu had holed up, they realised that he was hiding at the terrace, while his associates had fled, police said.
The local police was asked to surround the entire house and officials of the Special Cell team, including Khatri, climbed the terrace, said police.
Sonu was nabbed from there and as the police team was in the process of bringing him down from the terrace, which had no permanent stairs, Khatri lost his balance and fell, police said.
looked towards Khatri, they saw him falling down from the terrace. Khatri was rushed to Sanjay Gandhi Memorial Hospital, where he was pronounced dead on arrival, the official said.
When a crime team was sent to the spot to collect evidences in connection with the case, blood stains were found in the terrace, said the official.
From preliminary investigation, it seems that Khatri tripped over while he was supervising his team mates who were getting Sonu down the terrace. He fell from the terrace after receiving the bullet injury, the official said.
Khatri's body was sent for postmortem and his family was informed, police said.
Khatri played an active role in tracking down Sonu, a dreaded criminal, who had attacked a police constable while being chased in May 2015.
While Sonu is being interrogated, efforts to arrest his associates - identified as Pradeep Solanki, Pradeep alias Tota and Manjeet Mahal are in progress, Deep added.