I was down at the finish line (on the opposite side of the street) a couple of hours before the bombing watching a family member cross the finish line. My worry is that the crowd was so tightly packed that it might be tough to get any footage of what was happening at ground level. If I had simply looked down to see my feet, I wouldn't have been able to see them because we were so crammed in. If an object was on the ground, or if someone carried a backpack in at ground level, it seems like it could be missed by even the thousands of people snapping photos at all angles.

That being said, I'm wondering if you couldn't use some sort of facial recognition in conjunction with timestamps to track/detect unusual movements (like someone who quickly moves into the area of the detonation, then turns around and quickly moves out.)