The two men were  photographed walking together near the finish line of the marathon  before the explosions that killed three people and wounded about 180.  One of the two men was seen setting down a backpack at the site of the  second explosion "within minutes" of the blast, said Special Agent Rick  DesLauriers, the head of the FBI's Boston office.

"Somebody out there knows  these individuals as friends, neighbors, co-workers or family members  of the suspects," DesLauriers said. "And though it may be difficult, the  nation is counting on those with information to come forward and  provide it to us."

But he cautioned that the men should be considered armed and "extremely dangerous."

"No one should approach them. No one should attempt to apprehend them except law enforcement," he said.

The announcement capped a  day in which President Barack Obama brought a mixture of reassurance  and defiance to an interfaith memorial service in the city's Cathedral  of the Holy Cross. Whoever planted the bombs "picked the wrong city" to  attack, he said.

"Every one of us stands  with you," Obama told the crowd. "Boston may be your hometown -- but we  claim it, too. ... For millions of us, what happened on Monday is  personal."

Addressing the  still-unknown perpetrators, Obama added, "Yes, we will find you. And  yes, you will face justice. We will hold you accountable." And he looked  ahead to next year's race, predicting that "the world will return to  this great American city to run even harder and to cheer even louder for  the 118th Boston Marathon. Bet on it."

How we're affected as a nation by the bombing, other incidents

Among the crowd of about  2,000 were first lady Michelle Obama; the president's Republican  challenger last November, former Massachusetts Gov. Mitt Romney; the  state's current governor, Deval Patrick; and Boston Mayor Thomas Menino.  During an interlude, attendees were soothed by a performance by famed  cellist Yo-Yo Ma.

Menino praised each of  the three bystanders who were killed in the blasts -- Martin Richard,  described as a "young boy with a big heart"; Krystle Campbell, whose  spirit "brought her to the marathon year after year"; and Lingzu Lu, who  "came to the city in search of an education."

The audience also  included scores of police officers and other first responders. Crowds  erupted in cheers as the cathedral emptied out at the end of the  service, while others sang the national anthem.

How to help

Obama later stopped at a  high school to thank a group of first responders and volunteers, and  the first lady met with patients, families and hospital staff at Boston  Children's Hospital and Brigham and Women's Hospital, the White House  said.

Meanwhile, behind the  scenes, investigators were combing through surveillance video dating to  at least a week before the bombings to try to identify anyone who walked  the finish-line route before the race, said a source who receives  regular updates on the investigation.

Working leads

On Wednesday, a law  enforcement official who is being regularly briefed on the investigation  told CNN's Susan Candiotti that images showing two men near the  marathon finish line were being circulated to state and federal law  enforcement agencies. The source described the men as "possible  suspects."

But a source told CNN's Deb Feyerick on Thursday that those individuals are no longer of high interest to investigators.

The men faced scrutiny  and were considered potentially important to the case because one of  them had been carrying a black backpack close to one of the bombing  sites.

Asked about the images at a congressional hearing Thursday, Homeland Security Secretary Janet Napolitano urged caution.