Officials said the suspects Muhammad Shahid alias Bachunga and Muhammad Ali alias Ali Shooter confessed killing 12 people in different areas.
The suspects had killed three people in Baldia Town in 2008, police said.
They also admitted to have killed nine more in Mominabad, Qaimkhani Colony, Orangi Town and Pirabad.
The suspects are being further interrogated and will soon be produced before a court.
