The two successive blasts rocked the Boston Marathon finish line Monday afternoon.
And when the smoke cleared after the blasts in Boston's Back Bay section, dozens of victims lay in the street, some unconscious, some grievously injured, including some whose limbs had been torn off by the blast, Boston Globe reported.
The dead in the US' worst terror attack since 9/11 included an eight-year-old boy. Hospitals reported 144 people were being treated. Amongst them was a nine-year-old girl, a seven-year-old boy, a 12-year-old and a two-year-old at the Boston Children's Hospital.
Shaken witnesses described scenes of chaos and horrific injuries.
"It was just immediately (evident) there were injuries, right in the middle of the spectator crowds," said boston.com sports producer Steve Silva, who was on the scene to cover the race.
"There was blood everywhere; there were victims being carried out on stretchers. I saw someone lose their leg. People are crying. People are confused," Silva, who shot shocking video of the explosions was quoted as saying.
Oscar Otero was barely a few feet away.
"There was blood all over the place. I saw a leg, people with bones sticking out of their skin," he said.