According to government documents studied by The New York Times, the FBI asked several phone companies to analyze phone-call patterns of Americans using a technology called “communities of interest”. Verizon refused, saying that it didn’t have any such technology. AT&T, famously, did not refuse.

What is the “communities of interest” technology? It’s spelled out very clearly in a 2001 research paper from AT&T itself, entitled “Communities of Interest” (by C. Cortes, D. Pregibon, and C. Volinsky). They use high-tech data-mining algorithms to scan through the huge daily logs of every call made on the AT&T network; then they use sophisticated algorithms to analyze the connections between phone numbers: who is talking to whom? The paper literally uses the term “Guilt by Association” to describe what they’re looking for: what phone numbers are in contact with other numbers that are in contact with the bad guys?

When this research was done, back in the last century, the bad guys where people who wanted to rip off AT&T by making fraudulent credit-card calls. (Remember, back in the last century, intercontinental long-distance voice communication actually cost money!) But it’s easy to see how the FBI could use this to chase down anyone who talked to anyone who talked to a terrorist. Or even to a “terrorist.”

Here are a couple of representative diagrams from the paper:

Fig. 4. Guilt by association - what is the shortest path to a fraudulent node?

Fig. 5. A guilt by association plot. Circular nodes correspond to wireless service accounts while rectangular nodes are conventional land line accounts. Shaded nodes have been previously labeled as fraudulent by network security associates.