Erica Chenoweth on some of the strategic aspects of the bombing:
The blasts were near-simultaneous and co-located in a way that was sure to inflict maximal damage. One cannot necessarily assume intentionality from actions, but in this case the clear consequences of the bombs’ placement was to kill or wound many people — runners, spectators, medical personnel, law enforcement, and journalists alike. The three identified explosive devices near the finish line (two of which detonated, one of which did not) would have trapped people between the blasts, maximizing the casualties and creating a sense of being out of control.
The timing of the bombing – in broad daylight at the end of a symbolic event on a symbolic day — suggest that the perpetrators may have wanted to make use of the spectacle and media frenzy to increase the impact of the incident beyond the immediate casualties. No one has yet claimed responsibility for the attacks, though spectacular incidents like this may draw multiple claims of responsibility, as different groups may attempt to take “credit” for the mayhem.