ABC News' Boston affiliate WCVB reported surveillance video taken from cameras at Lord and Taylor along the marathon route were key to spotting the individual in question.
Senior law enforcement officials tell ABC News no suspect has been arrested yet, despite some early reports to the contrary.
Boston police and the FBI cancelled a scheduled afternoon press conference following an earlier bomb scare at the Federal Courthouse in Boston. An all clear was given about 30 minutes after the building was evacuated with a Code Red announcement. Boston police confirm a bomb threat was made and their investigation continues.
Law enforcement agencies had earlier pleaded for the public to come forward with photos, videos or any information that might help them solve the twin bombings. Police also gathered surveillance video from businesses around the finish line.
The bombs were made from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings to inflict maximum carnage, investigators and others close to the case said. But the FBI said nobody had claimed responsibility.
Investigators in white jumpsuits had fanned out across the streets, rooftops and awnings around the blast site in search of clues on Wednesday. They combed through debris amid the toppled orange sports drink dispensers, trash cans and sleeves of plastic cups strewn across the street at the marathon's finish line.
President Barack Obama branded the attack an act of terrorism. Obama plans to attend an interfaith service Thursday in the victims' honor in Boston.
Officials say 63 victims of the Boston bombing remained in hospitals, 12 of those in critical condition, and many with grievous injuries. Doctors who treated the wounded corroborated reports that the bombs were packed with shrapnel intended to cause mayhem. In addition to the 5-year-old child, a 9-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.
The trauma surgery chief at Boston Medical Center says most of the injuries his hospital treated after the marathon bombings 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."
Dozens of patients have been released from hospitals around the Boston area.
At Massachusetts General Hospital, all four amputations performed there were above the knee, with no hope of saving more of the legs, said Dr. George Velmahos, chief of trauma surgery.
"It wasn't a hard decision to make," he said Tuesday. "We just completed the ugly job that the bomb did."
The bombs exploded 10 or more seconds apart, tearing off victims' limbs and spattering streets with blood. The blasts near the finish line instantly turned the festive race into a hellish scene of confusion, horror and heroics.
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.
ABC News and The Associated Press contributed to this report.
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