Pressure cookers, possible activated remotely by cellphone, are believed to have been used to make the crude bombs that send deadly shrapnel hurling into the crowd of onlookers and competitors.  Doctors treating some of the 184 injured victims believe the explosives were packed with pellets, nails and sharp metallic objects.  Some patients had 40 or more such fragments embedded in their bodies.  The FBI has not commented on the nature of the two devices used in the blasts, but sources close to the investigation say shredded pieces of pressure cookers were found at the blast sites.