Medical personnel treating the wounded found evidence suggesting the bombmaker or bombmakers sought to maximize the suffering.
Dr. George Velmahos, head of trauma care at Massachusetts General Hospital, said his team found metal pellets and nails inside patients' bodies.
"They are numerous. There are people who have 10, 20, 30, 40 of them in their body, or more," Velmahos said.
While most of the patients treated at Brigham and Women's Hospital were wounded by "ordinary debris," three were struck by "perfectly round objects" that were uniform, consistent and metallic, the hospital's chairman of emergency medicine said.
Dr. Ron Walls also said one patient had more than 12 carpenter-type nails.
"There is no question some of these objects were implanted in the device for the purpose of being exploded forward," he said.
The process of healing is already well under way. Boston Medical Center has two patients in critical condition, down from 11 just after the bombings, Dr. Peter Burke, chief of trauma care, told reporters Wednesday. Ten patients are in serious condition and seven are in fair condition, he said.
A traumatic road ahead
Candace Rispoli was cheering on a friend when the festive atmosphere turned into a "terrifying hell."
"One of my best friends ... was actually trampled and thrown to the blood-splattered ground when it first happened," Rispoli told CNN's iReport. "She was able to get up and keep running, but when I turned around to seek all my friends, I could not see her and panicked."
Rispoli, who suffered minor injuries, said the attack has changed her life.
"I personally will never participate in an event of this nature in a city in fear that something like this could happen again," she said. "I keep replaying the moments of terror over and over in my head and am just still in utter shock. Always seeing terrible things of this nature happen all over the world on TV, my heart would always go out to those directly affected. But I never imagined in a million years I would be a spectator at the Boston Marathon running for my life."