Former senator Amarjeet Malhotra has been included in the list of suspects involved in the killing of 52-year-old Singh, who was the special assistant to Khyber Pakhtunkhwa chief minister with the status of provincial minister.
Senator Amarjeet Malhotra belongs to Tehsil Barikot of Swat district and has political affiliations with the ANP.
Main accused Baldev Kumar, who is in custody, also belongs to Tehsil Barikot.
Singh was shot dead on April 22 by contract killers allegedly hired by his miffed Hindu rival Baldev after he failed to secure a ticket for elections in 2013.
Baldev, a Swat district councillor and minority leader of Imran Khan-led Pakistan Tehreek-e-Insaf (PTI), hired contract killers by promising them Rs one million, police had said.
He was miffed at being overlooked for the ticket which was given to Singh who was comparatively new entrant to the PTI.
Pakistani Taliban had claimed responsibility for the murder