Time lapse taken following the Boston Marathon bombing

https://www.youtube.com/watch?v=aLjbN3Y40L0

The small package on the ground behind the blue fence in the lower left (right in front of the VIP section) is most interesting.  Note signalman at 2:02 as he backs up and flashes a light when he's in position.  There is an awful lot to be seen in 4:43 of time lapse video, including some of the "victims" as they exit the set.  Note that the ambulances were sequestered several blocks away and never deployed.