What's being tracked here? Language, culture, or history?
Google Ngrams viewer is a powerful tool, though its results are prone to error and open to interpretation. As the writer notes at the end of the article, this is just 'playing around' rather than hard data.
More terrorist N-grams | Fully (sic)
http://blogs.crikey.com.au/fullysic/2011/07/28/more-terrorist-n-grams/