Noor Mohammad died on the spot in his shop yesterday, they said.
A case of murder has been registered against six persons including Sagir, Mintu, Wakil, Shaista Begum, Praveen, Superintendent of Police Rakesh Jolly said.
Noor had filed a case against the accused after the gangrape incident on January 23 last year.
He was allegedly pressurised by the accused to withdraw the case, police said.