"You almost want to pick up the phone and say, 'Son, don't do this,'" the agent reflects. The teenager in question was Shifa Sadequee, a 19-year-old who was arrested on terrorism charges in 2006. Following a 2009 trial in which Sadequee represented himself, he was sentenced to 17 years in prison plus an additional 30 years of supervision.

The ethical issues involved in preventive counterterrorism cases like Sadequee's are the theme behind much of Homegrown. Following 9/11, law enforcement agencies were given a mandate to halt terrorist acts before they occurred, rather than investigate crimes after the fact. This directive inevitably gave rise to some disturbing ethical questions. When is it acceptable to arrest someone for a crime they haven't actually committed, but you think they might commit in the future? At what point do a teenager's online postings turn into a terrorism offense?