BOSTON — In a direct appeal for help from the public, the FBI on Thursday released pictures and video of two young men who officials believe may be responsible for the explosions that killed three people and wounded more than 170 on Monday during the Boston Marathon.
Officials said they had images of one of the men putting a black backpack on the ground just minutes before two near-simultaneous blasts went off near the finish line of the marathon at 2:50 p.m. Officials said that one video, which they did not release, showed the two men walking slowly away after a bomb exploded while the crowd fled.
At a news briefing here, Richard DesLauriers, the special agent in charge of the FBI's Boston field office, initiated the unprecedented crowdsourcing manhunt by urging the public to look at the pictures and video on the FBI's website, fbi.gov. The two men appear to be in their 20s, but DesLauriers did not characterize the appearance of the men or offer an opinion as to their possible ethnicity or national origin.
''Somebody out there knows these individuals as friends, neighbors, co-workers, or family members of the suspects," DesLauriers said firmly and grimly into the cameras. "Though it may be difficult, the nation is counting on those with information to come forward and provide it to us."
Almost immediately, calls started flooding the bureau's office complex in Clarksburg, W.Va. Traffic to the FBI's website spiked to the highest levels ever, an official said. For a brief period, the site was offline.
Typically about two dozen analysts sitting in cubicles in Clarksburg answer calls 24 hours a day, seven days a week. In the days after Monday's attacks, however, the center was inundated with calls and it has since increased the numbers of analysts and agents, according to a law enforcement official.
Attorney General Eric H. Holder Jr. and Director Robert S. Mueller III of the FBI were directly involved in the decision to release the images, a senior law enforcement official said.
Michael R. Bouchard, a former assistant director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, said that in releasing the pictures and video the authorities had taken a calculated risk.
''If you don't release the photos, the bad guys don't know you're on to them while you're looking," said Bouchard, who helped oversee the Washington, D.C., sniper case in 2002 and now runs his own security consultancy in Vienna, Va. "If you do release them, you run the risk they see them and change their appearance or go underground. The authorities made a calculated decision. The benefits of releasing the photos outweighed the risks of holding back and trying to identify them themselves."
He said several characteristics in the images selected for release were distinctive: the emblem on one man's hat, the backpacks they carried, their gaits, and the sight of them walking together.
''They don't know if these guys are from out of town, so they had to cast their net wider," said Bouchard, who said the widespread use of social media and cellphones made such identifications easier than just a few years ago."Now the public becomes a force multiplier."
In the Washington sniper case, he said, the culprits' car was spotted by a truck driver less than eight hours after photographs were made public.
At the briefing, DesLauriers did not specify what had led the FBI to call the two men suspects, but the official said that the decision was "based on what they do in the rest of the video." According to officials, when the blasts went off, most people fled in panic, but these two did not and instead walked away slowly, almost casually.
''We have a lot more video than what we released," the official said. "The sole purpose of what we released was to show the public what they looked like."
The fact that FBI officials chose to make the video images public suggested to some people familiar with law enforcement tactics that they have not been able to match them with faces in government photo databases, said Jim Albers, senior vice president at MorphoTrust USA, which supplies facial recognition technology to the United States. The FBI has a collection of mug shots of more than 12 million people, mostly arrest photos.
''The only conclusion you can reach is that they don't have a match they have confidence in," Albers said.
That could be a question of the quality of the images of the two suspects — the video footage posted by the bureau does not include high-resolution, frontal images of the two men's faces, as would be ideal for facial recognition software, Albers said. Or it may be that the search software, which produces a list of matches ranked by probability, simply did not find a persuasive match.
One law enforcement official said that the suspects in the images had captured the interest of the authorities because of their bags. Crime scene investigators recovered portions of a shredded black backpack that they believe carried explosives, this official said, and they were able to determine the brand and model of the bag. The backpack carried by at least one of the men in the videos appeared to be a match, the person said.
The briefing Thursday took place just a few hours after President Barack Obama spoke at an interfaith service of healing at Boston's Cathedral of the Holy Cross. Almost 1,800 people packed the pews and hundreds more outside listened intently as his words were broadcast into the morning sun.
His theme was the marathon, both as road race and metaphor, and he began his remarks with the same phrase that he used to end them: "Scripture tells us to run with endurance the race that is set before us."
He mourned the dead and assured the maimed that they were not alone.
''We will all be with you as you learn to stand and walk and, yes, run again," he said. "Of that I have no doubt. You will run again."
He spoke in personal terms. With a nod to his years as a student at Harvard Law School and to his speech at the 2004 Democratic convention here when he burst onto the national political stage, he embraced this heart-broken city as his own.
And whoever the perpetrators may be, Obama dismissed them as "small, stunted individuals who would destroy instead of build."
Mostly he rallied the living as he reflected this city's determined spirit.
''Like Bill Iffrig, 78 years old — the runner in the orange tank top who we all saw get knocked down by the blast — we may be momentarily knocked off our feet, but we'll pick ourselves up," the president said. "We'll keep going. We will finish the race."
If the perpetrators sought to intimidate or terrorize Boston, he said, "well, it should be pretty clear by now that they picked the wrong city to do it." The crowd cheered as if at a sports arena. "Not here in Boston."
The president connected with Boston's spirited sports fervor as he painted a more hopeful future in which "we come together to celebrate life and to walk our cities and to cheer for our teams when the Sox, then Celtics, then Patriots or Bruins are champions again — to the chagrin of New York and Chicago fans." This time next year, he said, "the world will return to this great American city to run harder than ever and to cheer even louder for the 118th Boston Marathon."
The mourners clapped enthusiastically and gave him a standing ovation; as he returned to his seat, he wiped away a tear.
The interfaith service where Obama spoke, "Healing Our City," brought together Christian, Muslim and Jewish religious leaders, as well as prominent state and local leaders. Former Gov. Mitt Romney, who was Obama's rival in last year's presidential election, was among the dignitaries at the service.
Boston's long-serving mayor, Thomas M. Menino, who recently announced that he would not seek a sixth term, rose from the wheelchair he has been using since he broke his leg last week and stood at the lectern to proclaim, "We are one Boston." He said he had never loved the people of his city more.
''And yes, we even love New York City more," he said to chuckles from the pews as he thanked Boston's rival for playing "Sweet Caroline," an unofficial Boston Red Sox anthem, at Yankee Stadium.
Gov. Deval Patrick praised the city for its resilience and its compassion. "In a dark hour," he said, "so many of you showed so many of us that darkness cannot drive out darkness, as Dr. King said; only light can do that."
After the service, Pauline M. DiCesare, 76, of Wayland, Mass., who grew up in Boston, remained in her pew.
''It was very uplifting, something we all need," she said as her voice cracked with emotion. "It's just the events of life. You're down and you get up again and life goes on, one step after another. Like the president said, the sun will rise tomorrow."
Outside the Gothic cathedral, Dina Juhasz, a nurse from Natick, Mass., who was at the marathon and helped to treat the wounded, said she appreciated the service.
''It's way too early for closure," she said. "It was a moment of acknowledgment to say, 'This was horrific and we are a community and we're going to get through this.' It's a beginning."
Contact the FBI with information at 1-800-CALL-FBI (800-225-5324) or bostonmarathontips.fbi.gov.