Slain Naseeruddin Haqqani was responsible for fund-raising, logistics and political affairs of the Haqqani Network, the network sources said.
Islamabad police confirmed the firing incident but did not say anything about identity of the man.
Eye witnesses said the slain man arrived in the market in his car with the driver to buy bread from a shop in a small market on Simly Dam road in Barakau.
A witness said the gunmen, riding motorcycles, arrived and started firing at him as he was waiting to buy bread from a shop.
A young man working in the shop was also injured when the gunmen sprayed dozens of bullets. He was taken to a hospital and is under treatment.
No one has claimed responsibility for the assassination.
