Good point winodog. Why were there so many people in that area with large backpacks, if they were just watching a race? It certainly made it much easier for the two triggermen to blend in with the crowd. The FBI needs to cast the net as wide as possible to determine if their were co-conspirators. Just because an individual did not carry the bomb does not mean they were not involved. The question is whether the FBI will be permitted to conduct a thorough investigation or whether they will be ordered to down tools once the 2nd suspect is taken out?
It's a university town and any university town with a younger population will have scores of people with backpacks.