The law enforcement officials have some of the bomb components but do not yet know what was used to set off the explosives, the source was quoted as saying.
The description of the devices, USA Today noted, coincides with observations earlier by Dr. George Velmahos, chief of trauma surgery at Massachusetts General Hospital, who said most of the injuries were "traumatic amputations", in which the blasts nearly blew off one victim's legs.
Velmahos had said that all of the victims had "10, 20, 30, 40 pieces of shrapnel embedded in their bodies, mostly in their legs, but as high up as their necks". He described the shrapnel as pea-sized pellets and nails stripped of their heads.
A law enforcement official told the newspaper earlier that the devices were believed to be assembled with gunpowder and ball-bearing-type material to serve as shrapnel. He described the devices as "rudimentary" but powerful.
It was unclear whether the devices were remotely detonated or included timers, the official was quoted as saying, adding that no conclusions had been drawn on whether an organised group or lone wolf had been responsible for the attack.
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