More than 170 people were hurt by the blasts, and doctors on Tuesday detailed some of the injuries, including broken bones, amputated limbs and head injuries.
"We've removed BBs and we've removed nails from kids. One of the sickest things for me was just to see nails sticking out of a little girl's body," said Dr. David Mooney, director of the trauma center at Boston Children's Hospital.
Two children remain in critical condition at the hospital with serious leg injuries. Mooney said that tourniquets applied by emergency responders at the race saved the children's lives.
Doctors at Massachusetts General Hospital also say they removed metal fragments from victims of the two bombs.
The explosives were contained in pressure cookers and hidden in black duffel bags on the ground, a person briefed on the investigation told The Associated Press on Tuesday. One of the explosives contained shards of metal and ball bearings, and another contained nails, the person said.
A second person briefed on the investigation confirmed that at least one of the explosives was made out of a pressure cooker. Both spoke on condition of anonymity to discuss an ongoing investigation.
Dr. Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Center said he saw an X-ray of one victim's leg that had "what appears to be small, uniform, round objects throughout it -- similar in the appearance to BBs."
Massachusetts General treated 31 victims of the bombs. The hospital performed four amputations and at least two more patients have legs that are still at risk of amputation, Dr. George Velmahos said.
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