Nova has been doing more and more of these “instant specials,” keyed to recent news events, and it’s a very smart move. Finding ways to explore the science behind headlines is a way in to stories about floods, hurricanes, tornadoes, blackouts – and, in this case, federal investigations of recent crimes. The April 15 bombing of the Boston Marathon is examined in tonight’s episode, showing in detail how investigators used some modern tools, and discarded others, to is