The FBI is seeking the public's help in identifying two Boston bombing… (Uncredited, AP )
BOSTON — The FBI appealed for the public's help Thursday in identifying two men wearing baseball caps and backpacks, one of whom was seen placing a backpack at the site of the second Boston Marathon bombing.
As President Obama traveled to the shaken city with a promise that it would "learn to run again," the FBI released photos and a video of two men seen walking through a crowd outside a restaurant near one of the two deadly explosions.
"They appear to be associated," Boston FBI chief Richard DesLauriers said as he detailed the most significant break yet in the investigation into who was responsible for the bombs that exploded Monday near the marathon's finish line, killing three people and injuring more than 170.
PHOTOS: Explosions at Boston Marathon
FBI officials believe they may have captured the planting of one of the crude pressure-cooker bombs outside a crowded restaurant near the finish line. They said an image showed the man — wearing a white baseball cap — depositing his backpack at the scene "within minutes" of the time the second bomb went off, DesLauriers said. The man walked back the way he came, away from the finish line, he said.
The same man is seen in surveillance footage walking a few steps behind a second man, this one dressed in khakis, a black jacket and a black baseball cap. That footage was captured at 2:37 p.m., about 13 minutes before the explosions, a few blocks away.
"We consider them to be armed and extremely dangerous. No one should approach them," DesLauriers said, urging members of the public with information to instead call the FBI's tip line.
"Somebody out there knows these individuals as friends, neighbors, co-workers or family members of the suspects," DesLauriers said of the agency's decision to tip its hand and release the photos.
"Though it may be difficult, the nation is counting on those individuals to come forward and provide [information] to us," he said. "No detail is too small. Each piece moves us forward towards justice."
The decision to release the photos was likely a gamble, a former FBI agent said.
"It's a cost-benefit analysis," said Richard Hahn, the former FBI senior supervisory agent in Long Beach. "Your perpetrator is going to see the photo if it's out there, and they could flee or go underground."
But, he said, the FBI may not be able to identify the individuals on its own and could decide that learning their identities through public help is worth the chance that they could try to evade law enforcement.
"If we don't know who that person is, we're not even at first base," Hahn said.
Sources close to the investigation also revealed that they have found what they believe to be the detonation system for the bombs — a circuit board and parts from a toy remote control vehicle.
Investigators have sorted through more than 3,000 images along with a massive amount of surveillance video; a department store camera near the scene of the second explosion produced some of the key images, officials have said.
"It's a long, tedious process — they're doing it as fast as they can," said Jason Pack, supervisory special agent at the FBI's national press office.
The revelations came as Obama traveled to Boston for the latest stop on a two-year-long road trip in which he has been called upon to comfort cities shocked by the effects of mass violence and disastrous weather.
"If they sought to intimidate us, terrorize us, to shake us from … the values that make us who we are as Americans, well, it should be pretty clear by now that they picked the wrong city to do it," Obama said to a standing ovation from more than 2,000 people at the Cathedral of the Holy Cross. "Not here in Boston."
Under the soaring expanse of the Gothic Revival cathedral, the president was called on to try to find new words of comfort equally eloquent to those he delivered after tragedies in Newtown, Tucson and Joplin.
Boston was different, he said. The city's long history of nurturing immigrants and the millions from around the world who have studied at its universities — including the president and his wife, both graduates of Harvard Law School — gave everyone a stake in the city, he said.
"Whether folks come here to Boston for just a day or they stay here for years, they leave with a piece of this town tucked firmly in their hearts," he said. "I know this because there's a piece of Boston in me.... For millions of us, what happened Monday is personal."
The service, which featured the melancholy strains of renowned cellist Yo-Yo Ma and a choir singing "America the Beautiful," drew several former Massachusetts governors, including former Republican presidential nominee Mitt Romney, who attended with one of his sons. Actor Bradley Cooper sat nearby, as did Teresa Heinz, wife of Secretary of State John F. Kerry, and U.S. Rep. Joseph Kennedy III.