investigators began digging for evidence. as work of the massive crime scene got under way, boston's police commissioner stood resolute. >> we will turn every rock over to find who is responsible for this. >> the criminal investigation pushed ahead painstaking work, the components of two bombs scattered across the streets for blocks. each particle would need to be found, cataloged, collected. 27 hours after the blasts, these crime scene photos began circulating showing pieces of the bomb, a partial timing device, and a possible clue. >> among items partially recovered are pieces of black nylon which could be from a backpack and what appeared to be fragments of bbs and nails possibly contained in a pressure cooker device.