They may be among the most memorable images of Monday afternoon: bystanders tying tourniquets around injured peoples’ legs, runners knocked off their feet, and a man whose pants were completely shredded, with only the waistband and cuffs remaining.
The placement of the two bombs on the ground along Boylston Street meant they would spray their deadly shrapnel and cause the most injuries to people’s lower extremities, according to an Army surgeon who saw similar injuries while treating soldiers injured by improvised explosive devices in Iraq and Afghanistan.
Numerous questions remain about the chemical makeup of the explosives within the bomb and exactly how they were triggered, but on Tuesday investigators announced some progress. They recovered parts of the bombs, enabling them to determine they were constructed out of two 6-liter pressure cookers packed with nails, metal, and ball bearings. A circuit board found in the area of the first blast was likely used to detonate the bombs, according to an official briefed on the investigation.
“[When the] bomb goes off, it doesn’t go straight up to the sky,” said Dr. Juan DeRojas, a surgeon who cared for soldiers on the front lines in Iraq and soldiers evacuated from Afghanistan and is now at Wilkes-Barre General Hospital in Pennsylvania.
Instead, he said, the blast radiates outward, sending shrapnel in all directions, meaning that people standing close by, who are at the greatest risk for injury, were most vulnerable at their lower extremity.
“It’s going off like the spokes of a wheel and the centerpiece is the bomb,” DeRojas said.
Law enforcement officials said Tuesday at a briefing that debris from the explosion was found on rooftops, giving a sense of its force.
Paul Gefken, a lab director who works on the effects of explosives at SRI International, a nonprofit research institute based in Menlo Park, Calif., said different parts of a bomb cause different kinds of damage. The pressure released by the explosion can cause ear or lung injuries, whereas shrapnel can cut through skin and sever arteries.
Lower-extremity injuries in adults might have translated into more full-body injuries in young children, Dr. David P. Mooney said, but that is not what happened for most of the eight patients treated at Boston Children’s Hospital.
Mooney, a surgeon and the hospital’s trauma director, said most of the injuries have been below children’s hips.
“There must be something about this blast,” he said.
DeRojas, however, said injuries may not have occurred above children’s hips because of where they were standing in relation to the bomb and other people. For example, small children may have been in their parents’ arms.
As investigators try to reconstruct how powerful the bombs were and what they were made of, the entire street, and even people who were injured, will likely become part of the reconstruction process, Gefken said.
By looking at damage to a structure or the types of injuries, such as perforated ear drums sustained by someone a certain distance away from the bomb, it is possible to reconstruct aspects of the device.Shelley Murphy, Chelsea Conaboy, and Andrew Ryan contributed to this story. Carolyn Y. Johnson can be reached at email@example.com. Follow her on Twitter @carolynyjohnson.