In what could be a major break in the case of the Boston Marathon, police are hunting a man seen in a department-store surveillance video dropping off a bag at the site of the bombings that killed three people.
Separately, a law enforcement official confirmed that authorities have found an image of a potential suspect but do not yet know his name.
Meanwhile, the third victim in the attack was identified as Boston University graduate student Lu Lingzi by The Shenyang Evening News, a state-run Chinese newspaper.
Yesterday’s developments - less than 48 hours after the attack that left more than 170 wounded at the world’s oldest and most prestigious marathon - marked a possible turning point in a case that has investigators analysing photos and videos frame by frame for clues as to who carried out the twin bombings - and why.
City council president Stephen Murphy, who said he was briefed by Boston police, said investigators saw the image on surveillance footage they got from a department store near the finish line, and matched the findings with witness descriptions of someone leaving the scene.
“I know it’s very active and very fluid right now - that they are on the chase,” he said. “They may be on the verge of arresting someone, and that’s good.”
The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings, and police 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”, Mr Murphy said.
At least 14 patients remained in critical condition. Dozens of patients have been released from hospitals around the Boston area, and officials at three hospitals that treated some of the most seriously injured said they expect all their remaining patients to survive.
Boston remained under a heavy security presence, and some people admitted they were nervous about moving about in public spaces.
Police were stationed on street corners across the city, while National Guardsmen set up tents on the Boston Common and stationed tactical vehicles.
Dr Horacio Hojman, associate chief of trauma at Tufts Medical Centre, 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 planned to visit Boston today to attend a service honouring the victims. The blasts also killed eight-year-old Martin Richard of Boston and 29-year-old Krystle Campbell of Massachusetts.