and a blood-stained ziber pull tab. another shows what looks likeead as deadly schaap knell. investigators are also combing through hundreds of photos from the scene, including this one, where a light-colored bag sits next to a mailbox. the before and after images capture its proximity to one of the two blast locations. authorities are scrutinizing every lead. the fbi needs help. after all, someone knows who did it. the person who did this is someone s friend, neighbor, co-worker, or relative. reporter: crime scene technicians also found gun powder residue. gun powder ignited inside the metal container, building pressure, which caused the container to explode, and an explosive expert explains to pentagon correspondent chris lawrence. of course in a crowd like this, it was so tightly packed with people, those people took the brunt of the explosion, and all the projectiles. reporter: a pressure cooker