The FBI is focused on people carrying bags and backpacks because they found evidence that a bag or backpack was used.
They are focused on people moving through the blast area before the explosion because, well, that's what bombers do when they place a bomb. Put it down, move along.
They are looking for people with bags, then without bags because non-bombers don't just leave their bags behind and bombers do.
They are looking for heavy sagging bags vs. light bags, which are different, because bombs are heavy and the bags they are carried in sag more than bags with light cargo. See here:
Gravitation - Wikipedia, the free encyclopedia
They are looking for people who look "different" because bombers are different: they are planting bombs while others are not.
But don't worry, I'm sure they are passing on potential suspects who are carrying the magic altoids and orange juice, who's fresh breath and healthy immune systems will make them impossible to detect and prosecute.