First, the danger of the “enemy within”—supposedly “homegrown” terrorists who are simultaneously alienated and radicalized, often with the help of the Internet. Second, the difficult task faced by law enforcement and intelligence agencies in uncovering such low-tech plots in advance. After all, with a 24 hour news cycle and an inevitable media feeding frenzy, even a relatively low number of casualties (say, compared with the civil war in Syria) can have a high impact. Especially given a target which is already an over-hyped media event.

Read more here