When the explosion happened, Arredondo told interviewers, he felt impelled to run to the aid of the injured, staying with one man even as other emergency personnel arrived and wheeled him away for treatment.
Another who rushed in to help was Vivek Shah, a local medical doctor who had finished running the race.
“I thought I would be one of the first people there [to help], because I was 25 yards away [from the explosion]," he told CNN Tuesday morning. Instead, the area was swarming with helpers by the time he started pitching in. “I’ve never seen anything like that.”
It's a theme that many are echoing in the wake of the attack.
"That's what Americans do in times of crisis," Suffolk County District Attorney Daniel Conley told "Good Morning America" on Tuesday. "We come together and we help one another. Moments like these, terrible as they are, don't show our weakness, they show our strength."
Within hours of the attack, a prayer gathering had been organized in the Boston suburb of Waltham, Mass.