Taken from a wider review of the 'Muslim 'Homegrown' Terrorism in the United States: How Serious Is the Threat?' which appears in the fall 2011 issue of International Security, from the Belfer Centre; with my emphasis:
In fact, the evidence suggests that engaging in terrorist activity in the United States carries a serious risk of exposure. First, although difficult to quantify, societal awareness about terrorism has grown considerably over the years. In October 2010, for example, members of a Hawaiian mosque reported to authorities a new member whose recent move to the area raised their suspicions. Contrast this with the hospitality and no-questions-asked reception that the Muslim community in San Diego gave to two of the September 11 hijackers in the months preceding the attacks. Additionally, over the past decade, alert citizens otherwise unacquainted with the would-be perpetrators have reported apparent terrorist activity involving American Muslims to authorities. Second, as the result of both a significant investment in grassroots counterterrorism efforts spanning the federal, state, and local levels as well as expanded prerogatives such as the availability of FBI assessments, would-be terrorists must contend with an increasingly sophisticated monitoring and investigative apparatus. Third, American Muslim communities have demonstrated a willingness to report aspiring terrorists in their midst—a dynamic that, according to several studies, has occurred in more than 20 percent of terrorism-related cases. Consider that Shahzad is the only homegrown Muslim terrorist unknown to authorities before he tried to execute his plot.
Link:http://belfercenter.ksg.harvard.edu/...ed_states.html