Law enforcement agencies have pleaded for the public to come forward with photos, videos or any information that might help them solve the case. Police also gathered surveillance video from businesses.
Multiple news organizations, citing unidentified sources, reported Wednesday that a video surveillance camera at a Lord & Taylor department store showed a suspect with a backpack near the finish line.
Investigators have said they believe the bombs were fashioned out of ordinary kitchen pressure cookers packed with explosives, nails, ball bearings and metal shards. They suspect the bombs were hidden in duffel bags and left on the ground.
The bombs exploded 10 or more seconds apart, tearing off limbs and spattering streets with blood.
Scores of victims remained hospitalized, many with grievous injuries. Fourteen were listed in critical condition. Doctors who treated the wounded corroborated reports that the bombs were packed with shrapnel.
The trauma surgery chief at Boston Medical Center said most of the injuries his hospital treated were to the legs.
"We have a lot of lower extremity injuries, so I think the damage was low to the ground and wasn't up,'' Dr. Peter Burke said. "The patients who do have head injuries were blown into things or were hit by fragments that went up.''
The blasts killed 8-year-old Martin Richard of Boston and 29-year-old Krystle Campbell of Medford. The Shenyang Evening News, a state-run Chinese newspaper, identified the third victim as Lu Lingzi. She was a graduate student at Boston University.