President Barack Obama promised a grieving Boston to hunt down whoever was responsible for the deadly bombing at the marathon while investigators focused on a man seen dropping off a bag and walking away from the site of the second of Monday's two explosions.
The discovery of the image - found on surveillance footage from a department store near the finish line - was detailed by a city politician two days after the attack that left three people dead and wounded more than 170. The footage has not been made public.
At an interfaith service honouring the victims, Mr Obama said "there is a piece of Boston in me" as he paid tribute to the city shaken by what he has called an act of terror. "Every one of us stands with you," he said.
There was a heavy police presence around the city's main Roman Catholic cathedral as residents lined up before dawn, hoping to get one of the roughly 2,000 seats inside.
Streets were blocked off around the Cathedral of the Holy Cross. Among the hundreds in line was 18-year-old Eli Philips. The college student was a marathon volunteer and was wearing his volunteer jacket. He said he was still shocked that "something that was euphoric went so bad".
The blasts killed eight-year-old Martin Richard, 29-year-old Krystle Campbell and Lu Lingzi, a Boston University graduate student from China.
Homeland security secretary Janet Napolitano said the FBI wants to speak with two men seen in at least one video from the marathon, but she added she is not calling them suspects.
Without providing details of the men's appearance or what the video shows, Ms Napolitano told the House Homeland Security Committee that "there is some video that raised the question" of two men. She said the investigation is continuing "apace".
Massachusetts governor Deval Patrick said he shared the frustration that the person or people responsible were still at large, but he said solving the case will not "happen by magic".
"It's going to happen by doing the careful work that must be done in a thorough investigation," Mr Patrick said. "That means going through the couple of blocks at the blast scene square inch by square inch and picking up pieces of evidence and following those trails, and that's going to take some time."