BOSTON — Plucking a couple of faces in baseball caps out of a swarming crowd, the FBI zeroed in on two suspects in the Boston Marathon bombing and shared surveillance-camera images of them with the world Thursday in hopes the public will help hunt them down.

The somewhat blurry but still detailed photos and video depict one young man in a dark cap and another in a white cap worn backward, both wearing backpacks and one walking behind the other on the sidewalk near the finish line as marathoners run by.

The man in the white hat was seen setting down a backpack at the site of the second explosion, said Richard DesLauriers, FBI agent in charge in Boston.

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

They looked much like typical college students, but DesLauriers described them as armed and extremely dangerous, and urged anyone who sees or knows them to tell law enforcement and “do not take any action on your own.”

The break in the investigation came just three days after the attack that killed three people, tore off limbs and raised the specter of another terrorist attack on U.S. soil. FBI photo-analysis specialists have been analyzing a mountain of surveillance footage and amateur pictures and video for clues to who carried out the attack and why.

The volume of information is likely to grow, joined now by a torrent of tips from people who think they might know the suspects. In releasing the images, the FBI gambled that useful clues will emerge, not just time-wasting leads.

Authorities are selective in putting out images of suspects because doing so risks tipping off the hunted and losing the element of surprise. But it can be a last resort when authorities hit a wall trying to identify or capture someone.

Within moments of the announcement, the FBI website crashed, perhaps because of a crush of visitors.

The images were released hours after President Barack Obama and first lady Michelle Obama attended an interfaith service at a Roman Catholic cathedral in Boston to remember the dead and the more than 180 wounded in the twin blasts Monday at the finish line of the 26.2-mile race.

The FBI video is a compilation of segments, altogether about 30 seconds long. The planting of the backpack, as described by authorities, was not part of the footage made public.

The man in the dark hat was dubbed Suspect 1 by the FBI and appeared to be wearing sunglasses. The other, in the white hat, was labeled Suspect 2. Both appeared to be wearing dark jackets. The FBI did not comment on the men’s height, weight or age range and would not discuss their ethnicity.

“It would be inappropriate to comment on the ethnicity of the men because it could lead people down the wrong path potentially,” said FBI agent Greg Comcowich, a spokesman for the Boston FBI office.

The enlarged pictures of white-hatted Suspect 2 in profile and head-on were blurry but still remarkable in their detail — and more revealing than those of Suspect 1.

While authorities said the information on the men began coming together over the previous day or so, agent Daniel Curtin said the FBI did not release the photos earlier because “it’s important to get it right.”

Distribution of the images brought both encouragement and unease to some Bostonians.

Jennifer Lauro of Topsfield, Mass., worried that the photos might breed fear and suspicion.

“It just looks like a college kid, so I think that’s going to make people feel vulnerable,” she said. “Because it could be anybody. It looks like any kid from Boston College or Boston University or any other school.”

Judy and Marc Ehrlich watched the marathon from a spot between miles 25 and 26 on Monday and felt the ground shake when the bombs exploded. The couple said it was creepy to see images of the suspects who were there at the same time, walking around. But they were comforted that the FBI had come up with suspects.

“Unless they kill themselves, they’re going to get found,” Marc Ehrlich said. He added: “There’s nowhere in the world to hide.”

James Kallstrom, who headed the FBI office in New York City in the 1990s, said “you get a million phone calls” when the public is asked for help. But “that’s why you have 1,000 people working for you.”

“The key is to have a good filtering system. There’s going to be a whole bunch of these things you just disregard,” he said.

At the Cathedral of the Holy Cross earlier in the day, Obama saluted the resolve of the people of Boston and mocked the bombers as “these small, stunted individuals who would destroy instead of build and think somehow that makes them important.”

“We will find you,” he warned.

Seven victims remained in critical condition. Killed were 8-year-old Martin Richard of Boston, 29-year-old restaurant manager Krystle Campbell of Medford, Mass., and Lu Lingzi, a 23-year-old Boston University graduate student from China.

The large volume of video and photos gathered as part of the investigation is being examined and enhanced by a special FBI unit. Investigators are looking at video frame by frame — a laborious process, though one aided by sophisticated facial-recognition technology and other software, forensic specialists said.

Investigators can set the software to search for certain types of objects or people matching a height and weight description. The software can also spot patterns that human analysts might not notice, such as a certain car that turns up in different places, said Gene Grindstaff, a scientist at Intergraph Corp., a Huntsville, Ala., company that makes video analysis software used by the FBI and other law enforcement agencies.

“Back in the days of 20 years ago, you were lucky if you had video and it was probably of poor quality and it took a tremendous amount of enhancement. Today you have a completely different issue,” Grindstaff said.

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