BOSTON -- Important clues to the Boston Marathon bombings are being collected from the victims themselves: the shrapnel doctors have removed from their injured bodies.
At Tufts Medical Center, doctors and nurses were cataloging all of the metal fragments removed from patients and turning them over to police and federal agents. Dr. William Mackey, chief of surgery, said the shards ranged in size from a few millimeters to about a centimeter.
At Massachusetts General Hospital, Dr. George Velmahos, chief of trauma surgery, said that doctors removed dozens of pellets, nails and other sharp objects from victims. Some of the items looked like "nails without heads," he said.
PHOTOS: Bombings splashed across nation's front pages
He left open the possibility that some of the metallic fragments had been kicked up from the street but said that “most of them are in the bomb.”
“There are people that have 10, 20, 30, 40 in their body or more,” he said. Some suffered serious burns.
Rep. Michael McCaul (R-Texas) said in a telephone interview that investigators believe the bombs were left in backpacks either inside or nearby trash cans.
Bomb experts are currently analyzing the materials used in the bombs to see if there is a signature, a style of explosive that would point to known terrorist organizations, he said.
He described the bombs as improvised explosive devices, or IEDs. It is not known whether foreign or domestic terrorists are behind the bombings.
McCaul suggested that the bombs were made in the United States, possibly by domestic radicals taking their cues from abroad. He pointed to a 2010 article in an extremist Internet magazine called Inspire.
In the article, headlined “Make a Bomb in the Kitchen of Your Mom,” Al Qaeda members based in Yemen encouraged Muslims in the United States to use household items to build explosives.
On Tuesday morning, officials said that three people had died and 176 were injured in the two explosions that occurred 13 seconds apart near the finish line of the marathon. Of the wounded, 17 were listed in critical condition.
VIDEO: Boston marathon explosion
At Tufts, a short walk from the marathon finish line, surgeons operated on eight victims, many of them with shrapnel injuries to their lower legs, doctors said. The injuries included fractures, nerve and muscle damage. Many lost large volumes of blood.
Ten patients remains hospitalized, but four were expected to be released by the end of the day Tuesday.
At Brigham and Women’s Hospital, nine patients underwent surgery and remained in critical condition, Walls said. One had a below-the-knee amputation and two others are still at risk of losing limbs, he said.
Dr. Mike Zinner, the head of surgery, said one patient suffered “very significant” head injuries. Another, who underwent successful surgery, had a penetrating neck injury.
Zinner said the trauma surgeons would probably not remove all the shrapnel from patients.
Live updates: Doctors remove pellets, nails from Boston victims
Boston bombing victim, 8, was watching for dad at the finish line
After Boston twin bombings, a nation offers its support and solidarity