Monday, 4 February 2013

Terrorist Networks - A look into the 9/11 tragedy

We have all been talking about social networks for quite some time now. Let us today look at a very interesting application of complex networks in real life, which could (and has) saved thousands of innocent lives. Well, I am talking about terrorist networks, and how complex networks can be used to foil their vicious intentions.

After the world went into shock due to the 9/11 attack, many internal information came into light. In early 2000, the CIA was informed of Nawaf Alhazmi and Khalid Almihdhar's links to al-Qaeda. They eventually became two of the 5 hijackers of American Airlines flight 77. When such an information comes out, the Intelligence uses their connections to uncover the entire network, rather than capturing the suspects immediately. I believe, the same was done in this case as well, but unfortunately, correct action was not taken in time.

Valdis Krebs studied the 9/11 hijacker network using publicly available information. While creating the network, Krebs faced three problems, which are inevitable in modelling any covert network:
  1. Incompleteness - Information purposefully not given by the investigators
  2. Fuzzy boundaries -  Difficulty in deciding whom to include and whom not to
  3. Dynamics - Such networks are always changing
He did a lot of background research on each of the 19 hijackers and built a trust network among them using open source information. This network had the hijackers as the nodes and edges were between those who had spent time living or learning together. This graph, the shape of a serpent, had a long mean path length of 4.75, in a network of less than 20 nodes. The network that he got was quite sparse, with many hijackers on the same flight being more than 2 steps away from each other. This contradicts intuition, as a planned task requires team mates to be close to each other. This, however, confirms with the terrorists' strategy to minimize damage - to ensure safety of others in case one is caught. This was also described in Osama bin Laden's infamous video tape found in Afghanistan. In the transcript, bin Laden mentions - "Those who were trained to fly didn't know the others. One group of people did not know the other group".

Then, Krebs remodeled the graph by adding new edges among the nodes who attended meetings together. Such meetings are held to coordinate among distant points and discuss progress. After such meetings, the ties go dormant again till activity demands. He added six edges (meetings) to the graph and the mean path length dropped to 2.79. In another graph that also included immediate correspondences, Mohammed Atta, the ring leader of the conspiracy, was shown to have the maximum degree and closeness centrality, but not betweenness centrality.

Two important conclusions that Krebs drew out :
  1. All 19 hijackers were within 2 steps of the two original suspects uncovered in 2000
  2. Social network metrics reveal Mohammed Atta emerging as the local leader
The study also pointed that important links were concentrated around the pilots. They made the perfect "take-out" points, by having unique skills and connections.

Such study clearly shows the emerging importance of social network analysis on such terrorist networks. In fact, the arrest of Canadian terror cell in June 2006 and a plot to blow U.S. passenger planes crossing the Atlantic Ocean in August 2006 was foiled using techniques similar to those in network analysis. Questions were raised on the U.S. Intelligence on why they could not stop this catastrophe from happening. After all this was their job, isn't it? But it is not so easy dealing with a network that focuses on secrecy and stealth.

Such covert networks behave quite differently as compared to social networks. In social networks, strong ties reveal close clusters. This is not true in case of covert networks. Strong ties, that may have been formed in training camps, keep the cells interconnected, but these mostly remain dormant and make these ties appear as weak ties. These links become active only when absolutely necessary. As the target event approaches, the network activity increases - the flow increases across known links, new links are formed, the cash flow reverses, etc. When these activities seem similar to the known patterns closer to disaster, it is time "cut out the middleman".

[1] Valdis E. Krebs. Mapping networks of terrorist cells.
[2] P. V. Fellman, R. Wright. Modelling Terrorist Networks.
[3] United States Department of Defense. 2001. Transcript of bin Laden Video Tape. December 13.
[4] Wikipedia

Image courtesy:
Valdis E. Krebs. Mapping networks of terrorist cells.

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