The problem with committing a crime and/or alleging a crime in the modern age is that at any significant event there is an ocean of mobile new media recording everything from most any possible angle and so is the case with the bombing in Boston.
There are several sets of photo’s that are circulating around the net. One is of a Caucasian young man with dark brown hair who is wearing a brown hoodie. In pictures before the blast he has a dark blue or black backpack. Later he is shown without his backpack (insert ominous music here).
There is also a man with black hair with a receding hairline who appears to be olive skinned and between 35-45 years of age. He is seen carrying a large black tool-bag or sports-bag (insert ominous music here).
Now read carefully. The problem is that at a sporting event such as a marathon many people would have such a sports duffel bag, especially at the finish line.
Dozens of people would have a bag of water, supplies and a change of clothes waiting for runners to cross the finish line. The runner would take the bag inside and go find a place to freshen up. So people with a bag before and without a bag later would be a common occurrence.
Pointing fingers based on the “bag technique” could get someone hurt or even killed.