Finding the Terrorists
The network of phone calls in the United States is very extensive, containing a complex system of links too large to express in a tractable manner on a sheet of paper. Such a network contains a node for each American who has used a phone and an edge between every pair of people who have called each other. Imagine everyone you have called in the past few years, everyone they have called in the past few years, and so on. Due to its immenseness, a data set containing millions of Americans and their phone calls may be difficult to analyze. However, given the principles of graph theory, the NSA has a method to search through an extensive list of call records and filter out the people who may be terrorists. Afterward, the NSA can then monitor the potential terrorists more closely and thwart malicious plots.
The method the NSA uses is akin to breadth-first search. The search begins with two known suspects who have communicated with each other. The next step is to consider those who have called each of these two suspects. However, because not everyone who communicated with these people is a terrorist, a filter is used. This filter applies the principle of triadic closure which, in the context of phone calls, states that if person A regularly communicates with person B and person C, then B and C have an increased chance of communicating. In a strongly integrated terrorist network, such communication triangles have likely formed. With this concept, the NSA homes in on the people who have communicated with both of the known suspects, not just one of them. The next step is to search for records of communication between the suspects’ contacts and for others who have communicated with the suspects’ contacts. Again, people in communication triangles are added to the list of potential terrorists. In this manner, the NSA can map out a large portion of the terrorist network.
After amassing enough evidence that the graph generated is indeed a terrorist network, the NSA can again use graph theory to determine a strategy to dismantle the criminal group. Local bridges and highly integrated nodes are crucial for the structural integrity of the communication network between members. Therefore, officers can focus their efforts on arresting people who are part of local bridges and people who form many connections with others. When these people are in jail, the terrorist network loses crucial members and connections. The NSA can then more easily stop the insidious plans.
http://www.forbes.com/sites/gregsatell/2013/06/07/how-the-nsa-uses-all-those-verizon-phone-records/