Casting for additional leads, authorities pleaded Tuesday with spectators from the Boston Marathon to send photos and video that may shed light on who set off two shrapnel-studded bombs that killed three people and injured at least 176.
"There has to be hundreds, if not thousands, of photographs, videos and other observations that were made down at that finish line yesterday," said Timothy Alben, superintendent of the Massachusetts State Police. "You might not think it's significant, but it might have some value to this investigation."
Informal public efforts online sprang up almost immediately to scour the vast collection of photos and video already posted on Twitter, Facebook and other sites in the wake of the attack. In an era of ever-present smartphones, race fans might be holding evidence without even knowing it.
Authorities confirmed at a mid-morning news conference that only two bombs went off Monday. Contradicting reports from the chaotic early hours after the blasts, they said that no unexploded devices were found.
Overnight, law enforcement officials searched a suburban Boston apartment building, interviewed a man and removed a duffel bag and two trash bags. They also issued alerts for a rental van and a man seen leaving the scene of the blasts.
But the significance of the police activity, if any, was not clear one day after the twin blasts, which detonated seconds apart and turned the finish line of the marathon into a hellish scene of panicked spectators, shattered glass and blood-spattered sidewalks.
Law enforcement officials told NBC News that the bombs were packed with ball bearings and BBs, apparently intended to increase the casualties.
Among the dead was an 8-year-old boy, identified by NBC affiliate WHDH as Martin Richard, who was waiting at the finish. Among the injured were brothers, 33 and 31, who each lost a leg from the knee down, The Boston Globe reported.
As of 9 a.m. ET, there were 125 patients from the blasts being treated at six Boston hospitals, including some who had injuries described as "limb-threatening." An official at Boston Children's Hospital told the Globe that the youngest victim was 3 years old.
Investigators were studying surveillance video to look for anyone placing packages at the points where the bombs exploded. Law enforcement officials said there was video showing a person, from the back, carrying two backpacks, but they said it was too soon to know whether that was related to the attack.
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