With no credible claim of responsibility, and no arrests of any suspects, authorities took pains Tuesday to stress that their investigation remained wide open. From President Barack Obama on down, they suggested that the bombings could be the work of a foreign or domestic group, or of an individual.
Investigators gathered an array of surveillance tapes from businesses near the attack site, intending to study through them frame by frame. Police also exhorted marathon spectators to share any video or photos they took in the hours before and just after the attacks — hoping for some clues about how and when the bombs were placed.
"There has to be hundreds, if not thousands, of photos and videos" that might aid investigators, said state police Col. Timothy Alben.