decade in hiding. our senior international correspondent stan grant investigates a very old and very shady way of exchanging cash in pakistan. >> reporter: branded terrorist banking. doesn't take long important people to get nervous. we are outside the uncomfortably named but unconnected osama money exchange. these meant don't want us here. we can't say we are hiding anything. according to those that know, you want to trace the black money trail, this is the time and place to start. this is the sort of place where these transactions are carried out. >> we can see this type of place, i told you. >> reporter: he's a pakistani academic who tracks the workings of al qaeda. he zeroed in on osama bin laden's financing. bankrolling, terror network, he says, tens of millions of dollars each year. how to do it? something called -- banking.