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(CNN) — The FBI on Thursday released photos and video of two men it called suspects in the deadly bombings at the Boston Marathon and pleaded for public help in identifying them.

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.”

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