afternoon near the finish line of the world's oldest international foot race.
"This is how bad this is," wrote Boston Globe columnist Kevin Cullen.
"I went out (on) Monday night and bumped into some fire-fighters I know. They said one of the dead was an 8-year-old boy from Dorchester who had gone out to hug his dad after he crossed the finish line."
"The dad walked on; the boy went back to the sidewalk to join his mom and his little sister. And then the bomb went off. The boy was killed. His sister's leg was blown off. His mother was badly injured."
Just days earlier, organisers of the Boston marathon said they were dedicating the 26-mile marker to the 20 children and six educators shot and killed at the Sandy Hook Elementary School in Newtown, Connecticut, four months ago.
Those children - gunned down by a 20-year-old with an assault rifle who had earlier killed his mother and who went on to take his own life - were six and seven years old.
Marathons are 26.2 miles long, or a tad under 42.2kms, so the 26-mile marker would have been just 1,056 feet (nearly 322 meters) from the finish line.
Overnight and into Tuesday, surgeons performed amputations on many of the injured, although an overall figure from all Boston area hospitals was not immediately available.
"If I have my numbers right, we have performed four amputations" at Massachusetts General Hospital, its chief of trauma surgery, George Velmahos, told reporters.
"There are two more limbs that are at risk, but I hope that we will save those legs... This bomb obviously was placed probably low on the ground, and therefore lower extremity injuries are to be expected."
Velmahos said he knew of patients at his hospital as young as 28 and as old as 71.
At Boston Children's Hospital, meanwhile, one of eight children admitted after the blast was reportedly a two-year-old with a bleeding head injury.
Liz Norden, a mother of five, told the Boston Globe how two of her sons had lost a leg in the blast. Both had gone to Boylston Street to see a friend finish the race.
She said she was taking groceries into her home in Wakefield, north of Boston, when her phone rang. She picked up to hear her 31-year-old, in an ambulance, say: "Ma, I'm hurt real bad."
He didn't immediately know where his 33-year-old sibling was, but in the ensuing hours the terrible news emerged - both lost a leg, from the knee down. The two had been taken to separate hospitals.
Lucky to be alive and unhurt was avid runner Bill Iffrig, 78, of Lake Stevens in Washington State, who fell onto his knees as he neared the finish line and the first blast went off to his left.
He told his local newspaper, the Everett Herald, that he briefly saw an object that resembled a coffee can, which he thinks might be a remnant from the explosive device.
"Then all this smoke was coming from someplace, and I wasn't able to see too much," said Iffrig, who amid the melee got back up on his feet and ran the roughly final four yards to complete the race.