Deepak Bhardwaj, the local BSP leader who was the richest candidate in 2009 Lok Sabha polls, was shot dead inside his farmhouse in south Delhi's Rajokri on March 26 by three men who came in a car.
Sources said they have detained a man, who they suspect was behind the wheels of the car in which the shooters escaped.
"We are investigating the matter. We are questioning several people," a senior police official said.
Investigators are also looking into the role of an insider who might have tipped off the killers about the movement of Bhardwaj and the topography of the farmhouse.
Around 40 employees were inside the around 25 acre farmhouse at the time of the incident. However, the two Personal Security Officers were not with Bhardwaj or inside the farmhouse at the time of the incident.