Officials tell CBS News that the bombs detonated at the Boston Marathon finish line on Monday were low-grade, improvised explosive devices containing nails and ball bearings. Doctors treating the wounded also report removing metal shrapnel, including BB pellets, from the victims. The bombs were made to look like discarded property, though it is still unknown whether they were placed in trash cans or on the sidewalk, officials say. CBS’s Charlie Kaye also reports that one of the devices, found near a Starbucks, may have been in a metal pressure cooker. The bombing killed three people and injured 176.