The New York Times cited Republican Michael McCaul, who heads the House Homeland Security Committee, as saying authorities believe the explosive may have been a “pressure-cooker device”, similar to improvised explosive weapons that have been used against American troops in Iraq and Afghanistan.
McCaul, a former federal prosecutor who received briefings Tuesday morning from the FBI and Homeland Security officials, also said that authorities still did not know whether the attack was a foreign or domestic plot.
An explosive device fashioned from a pressure cooker was also used in the attempted bombing of Times Square in New York in 2010, the Times noted.
A USA Today report citing an unnamed source said that the explosives were put in six-litre pressure cookers, placed in black duffel bags and left on the ground. They were packed with shrapnel.
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.