Rasterfahndung - the mother of NSA prism

The German BKA used the Rasterfahndung-technique in the 1970's to search for RAF terrorists.

Basically it is to brainstorm for descriptive attributes that fit the wanted person. And then use all available information sources, like phone books or customer lists, that help filter out persons that match.

1979 the BKA in fact knew, that the terrorist were living in a rented appartement under false name, somewhere near Frankfurt.
Also they assumed that they have to pay the electricity bill in hard cash (anonymous).
As 18'000 bills were payed by hard cash, the investigators started shrinking the list by removing persons that really exist in other. So if one of the names also exists in an other register, it most possibly isn't a suspicious person and can be removed from the list. Following registers were used:
- car owners
- registered residents
- pensionists
- social welfare benefit receivers
- property owners
- fire insurance owners
- public health care owners

Till there remained only 2 persons!