The FBI on Thursday released surveillance video showing two potential suspects in the Boston Marathon bombing.
FBI Agent Richard DesLauriers says the photos came from surveillance cameras near the explosion sites.
Investigators had been focusing on a man wearing a white baseball cap seen dropping off a bag, and then walking away from the site of the second of two deadly explosions. CBS News correspondent Bob Orr reports that investigators were able to retrace his steps and discovered video of the man with another man carrying a backpack.
The surveillance footage was released three days after the attack that left three people dead, wounded more than 170, and cast a dark shadow over one of this city's most joyous traditions.
Orr reports that after releasing the video, the FBI is hoping for tips that will identify men. Investigators are using facial recognition software; scouring criminal databases; and trying to to find a match. They have some potential names for these suspects but they are currently unable to match
Earlier Thursday, President Barack Obama paid a visit under heavy security to offer reassurance to the city and a warning to those responsible for the attack: "We will find you."
At an interfaith service honoring the three people killed and more than 180 wounded in Monday's twin blasts, 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 those who couldn't get a ticket 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.
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.
Associated Press contributed to this report.