John Mueller, Professor of Political Science at Ohio State University and the author of "The Remnants of War", in an article at Foreign Affairs, explores the overstated nature of the terror threat. His article,
Is There Still a Terrorist Threat?, asks:

"But if it is so easy to pull off an attack and if terrorists are so demonically competent, why have they not done it? Why have they not been sniping at people in shopping centers, collapsing tunnels, poisoning the food supply, cutting electrical lines, derailing trains, blowing up oil pipelines, causing massive traffic jams, or exploiting the countless other vulnerabilities that, according to security experts, could so easily be exploited?

One reasonable explanation is that almost no terrorists exist in the United States and few have the means or the inclination to strike from abroad. But this explanation is rarely offered."

He also has a book coming out in November, called Overblown: How Politicians and the Terrorism Industry Inflate National Security Threats, and Why We Believe Them.