BOSTON — Investigators in the Boston Marathon bombing pressed the search Thursday for one or more potential suspects spotted on video, while President 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.”

In Washington, Homeland Security Secretary Janet Napolitano said the FBI wants to speak with individuals seen in at least one piece of footage from the race, but she said she isn’t calling them suspects. She gave no details on what the video shows.

Additional Photos

Investigators on Thursday inspect the roof of a building across the street from the area where a bomb exploded near the Boston Marathon finish line, AP

President Barack Obama attends the "Healing Our City: An Interfaith Service" at the Cathedral of the Holy Cross in Boston on Thursday. The service was dedicated to those who were gravely wounded or killed in Monday's bombing. AP

The FBI and other law enforcement officials are planning a news briefing Thursday afternoon on the investigation.

The briefing will be held at 5 p.m. at the Sheraton Boston Hotel.

A day earlier, City Council President Stephen Murphy said investigators were hunting for a man seen on a department store surveillance video dropping off a bag near the finish line and then walking away.

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.

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.

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.