The deceased has been identified as Vipin, 24, who was the driver and aide of former INLD legislator Singh, said police.
The incident was reported at around 5.45 PM when a car intercepted Vipin's motorbike and around four men got off the vehicle who overpowered Vipin and allegedly started to thrash him.
When he tried to run away, they chased him and allegedly shot him multiple times, said a senior police official citing eyewitnesses.
Police said they came across a few eyewitnesses who told them that Vipin was riding alone. The total number of assailants was reported to have been four. However, these details are yet to be verified, said the official.
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The scene of the crime -- a desolated stretch with no buildings or installations -- is such that it is difficult to to find any CCTV footage, the official added.
It is suspected that the murder was planned in advance and the location was part of the plan, he said.
"His relatives maintain that Vipin had received no threat," said DCP (Outer) Vikramjit Singh.
Former INLD legislator from Najafgarh, Bharat Singh, was shot dead by assailants in March this year following which six persons, including the alleged mastermind, identified as one Udaiveer alias Kaale, and Nitin Sherawat, the son of a former Delhi MLA, were arrested by Crime Branch.
Days before Bharat Singh's murder, some unidentified men
had allegedly picked up Vipin and thrashed him before he managed to escape, said a police official.
Believed to be the prime aide of Bharat Singh, Vipin used to accompany him everywhere. He had also campaigned for Bharat Singh during the 2015 Assembly elections, which the former MLA had contested unsuccessfully as an Independent candidate.
Vipin is also believed to be associated with Singh's brother Kishen Pehelwan, a former gangster, since he was a teenager, said the official.
Vipin lived with his family in southwest Delhi's Janta Vihar area, close to Najafgarh. Police are yet to ascertain as to whom he had come to meet in or around Rohini.
Also, the records of all the accused involved in Singh's murder are to be checked to ascertain if any of their associates had tried to contact Vipin in the past few months, the official added.
According to police, the killing of Bharat Singh was an act of vengeance carried out jointly by different gangs. Gangs of criminals who had at some point in the past been rubbed the wrong way by the former legislator and his gangster brother Kishen Pehelwan allegedly came together to kill him, police had said.
The absconding accused, Hemant, had provided weapons and other logistics for the attack in March.
Meanwhile, police said that the search for Hemant has been intensified after today's incident. The Crime Branch of Delhi Police, which had cracked the case of Singh's murder, are also likely to be involved in the probe into Vipin's murder, police added.