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Aprils deadly marathon bombings. the accused bombers had initially chosen july 4th as their day to strike. >> youll see troopers and national guard out there at security checkpoints. and the camera security system obviously is another layer of that. >> reporter: the first big event in boston since the marathon. the secure area is bigger, and knapsacks like the ones used by the bombers will not be permitted. in los angeles, police are stepping up their visibility and law enforcement in the nation...