BOSTON -- Investigators in the Boston Marathon bombing pressed the search Thursday for one or two potential suspects spotted on video, while President Barack Obama paid a visit under heavy security to offer words of reassurance to the city and a warning to those responsible for the attack: "We will find you."
Mountains of marathon footage yielded a possible breakthrough as investigators zeroed in on a man seen on department store surveillance video dropping off a bag near the finish line and then walking away, City Council President Stephen Murphy said on Wednesday.
In Washington, Homeland Security Secretary Janet Napolitano said Thursday that the FBI wants to speak with individuals seen in at least one video from the race, but she said she isn't calling them suspects in the twin bombings Monday that killed three people and wounded more than 170. She gave no details on what the video shows.
At an interfaith service honoring the victims, the president sought to inspire a stricken city and comfort an unnerved nation, declaring that Boston "will run again."
"We may be momentarily knocked off our feet," Obama said at the Roman Catholic Cathedral of the Holy Cross. "But we'll pick ourselves up. We'll keep going. We will finish the race."
The crowd applauded as Obama warned those who carried out the attack: "Yes, we will find you. And, yes, you will face justice."
There was a heavy police presence around the cathedral as residents lined up before dawn, hoping to get one of the roughly 2,000 seats inside. By 9 a.m., they were being turned away.
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."
Ricky Hall of Cambridge showed up at 8 a.m. but was turned away from the line to get inside that was already stretching down at least two city blocks.
"I came to pay my respects to the victims," he said. He said he was also angry that someone would desecrate the marathon, and he urged maximum punishment for the perpetrators.
Massachusetts Gov. Deval Patrick said he shared the frustration that those 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," 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."
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. Investigators had appealed to the public to provide videos and photographs from the race finish line.
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. He said he was briefed by Boston police.
Separately, 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.
Seven bombing victims remained in critical condition.
Dr. Peter Burke, chief of trauma surgery at Boston Medical Center, said Thursday that one of the youngest victims, a 5-year-old boy is getting better and "is going to be OK." A blast can often compress a child's chest, bruising the lungs and heart, he said, adding he is pleased with the boy's progress.
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.
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.