Here is video footage of the moment two explosions hit the Boston Marathon - leaving three dead and more than 100 injured.
The focus in Boston right now is finding out who is responsible for the blasts.
Former CIA agent Robert Grenier says the significance of when the attack happened could be a clue.
"One wonders if perhaps there might not be a domestic right-wing terrorist component to this," he told Al Jazeera.
Watch a highlight of the interview and amateur footage of the second explosion here.