Near the finish line, Brighid Wall of Duxbury, Mass., stood to watch the race with her husband and children, cheering on the competitors laboring through the race's final demanding steps.
In the post-race chute Tracy Eaves, a 43-year-old controller from Niles, Mich., proudly claimed her medal and a Mylar blanket, and took a big swig from a bottle of Gatorade.
And at the corner of Newberry Street and Gloucester, cab driver Lahcene Belhoucet pulled over, relishing the overabundance of paying passengers on an afternoon that traditionally gives almost as much of a boost to Boston's economy as it does to the city's spirits.
But the blast — so loud it recalled the cannon fire heard on summer nights when the Boston Pops plays the 1812 Overture — brought the celebration crashing down.
"Everyone sort of froze, the runners froze, and then they kept going because you weren't sure what it was," Wall said. "The first explosion was far enough away that we only saw smoke." Then the second bomb exploded, this time just 10 feet away.
"My husband threw our kids to the ground and lay on top of them," Wall said. "A man lay on top of us and said, 'Don't get up! Don't get up!' "
From her spot beyond the finish, a "huge shaking boom" washed over Eaves.
"I turned around and saw this monstrous smoke," she said. She thought it might be part of the festivities, until the second blast and volunteers began rushing the runners from the scene.
"Then you start to panic," she said.
Back in the field, Jones-Bolton noticed runners turning around and coming back at her. Then she realized most were wearing the blankets given to those who'd already completed the race. Suddenly the race came to halt, but nobody could say why. When word began to spread, Jones-Bolton panicked at the thought of her husband standing at the finish line, but was reassured by other runners.
At the finish, Wall, her husband and children raised their heads after a minute or two of silence. Beside them, a man was kneeling, looking dazed, blood dripping from his head. A body lay on the ground nearby, not moving at all. But in a landscape of blood and glass and twisted metal, they were far from alone.
"We grabbed each other and we ran but we didn't know where to run to because windows were blown out so another man helped me pick up my daughter," and they ran into a coffee shop, out the back door into an alley and kept going.
Meanwhile, the instincts of Dr. Martin Levine, a Bayonne, N.J., physician who has long volunteered to attend to elite runners at the finish line, told him to do just the opposite. Looking up at the plume of smoke, he estimated it was about two storefronts wide and quickly calculated how many spectators might be located in such an area.
"Make room for casualties — about 40!," he yelled into the runners' relief tent. "Get the runners out if they can!" And he took off. Just then the second bomb went off. He reached the site to find a landscape resembling a battlefield, littered with severed limbs.
"The people were still smoking, their skin and their clothes were burning," he said. "There were lower extremity body parts all over the place ... and all of the wounds were extreme gaping holes, with the flesh hanging from the bones — if there was any bone left."
Back in his cab, Belhoucet said he mistook the first blast for an earthquake. Fearing that a building might collapse, he considered running. But then people came pouring down the street and he beckoned a family into the car. He grabbed the wheel, then turned momentarily to ask where they wanted to go.
Only then did he notice the man's face, dripping with blood.
Now, three days after the bombing, investigators had made significant headway in deciphering the method behind the terror.
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