The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings, investigators and others close to the case said.
Investigators suspect the devices were then hidden in black duffel bags and left on the ground.
As a result, they were looking for images of someone lugging a dark, heavy bag.
One department store video "has confirmed that a suspect is seen dropping a bag near the point of the second explosion and heading off," Murphy said.
A law enforcement official who was not authorized to discuss the case publicly and spoke to The Associated Press on the condition of anonymity confirmed only that investigators had an image of a potential suspect whose name was not known to them and who had not been questioned.
Several media outlets reported that a suspect had been identified from surveillance video taken at a Lord & Taylor department store between the sites of the bomb blasts.
The turn of events came with Boston in a state of high excitement over conflicting reports of a breakthrough.
A law enforcement official briefed on the investigation told the AP around midday that a suspect was in custody. The official, who was not authorised to divulge details of the investigation and spoke on the condition of anonymity, said the suspect was expected in federal court. But the FBI and the US attorney's office in Boston said no arrests had been made.
By nightfall, there was no evidence anyone was in custody. No one was taken to court. The law enforcement official, who had affirmed there was a suspect in custody even after federal officials denied it, was unable to obtain any further information or explanation.
At least 14 bombing victims, including three children, remained in critical condition. Dozens of victims have been released from hospitals, and officials at three hospitals that treated some of the most seriously injured said they expected all their remaining patients to survive. A 2-year-old boy with a head injury was improving and might go home Thursday, Boston Children's Hospital said.
On Wednesday, investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues. They picked through trash cans, plastic cup sleeves and discarded sports drink dispensers.
Boston remained under a heavy security presence, and some people admitted they were nervous about moving about in public spaces.
Tyler King, a personal trainer from Attleboro who works in Boston, said four of five clients canceled on him a day earlier because they were worried about venturing into the city. He took the train in, but "I kind of kept my head on a swivel".
Kenya Nadry, a website designer, took her 5-year-old nephew to a playground.
"There's still some sense of fear, but I feel like Boston's resilient," she said. "The fine men in blue will take care of a lot of it."
Police were stationed on street corners across downtown Boston, while National Guardsmen set up tents on the Boston Common and stationed tactical vehicles.
Dr. Horacio Hojman, associate chief of trauma at Tufts Medical Center, said patients were in surprisingly good spirits when they were brought in.
"Despite what they witnessed, despite what they suffered, despite many of them having life-threatening injuries, their spirits were not broken," he said. "And I think that should probably be the message for all of us - that this horrible act of terror will not bring us down."
President Barack Obama and his challenger in the last election, former Massachusetts Gov. Mitt Romney, planned to visit Boston on Thursday to attend a service honouring the victims.
Obama signed an emergency declaration for Massachusetts on Wednesday and ordered federal aid to supplement the local response to the bombings.
The blasts killed 8-year-old Martin Richard, of Boston, 29-year-old Krystle Campbell, of Medford, and Lu Lingzi, a Boston University graduate student from China.
- AP
