The FBI released dramatic photographs and video Thursday of two suspects in the Marathon bombing, plucking the young men out of the obscurity of the festive downtown crowd and putting their faces on worldwide display as they enlisted the public’s help in identifying them.

“Somebody out there knows these individuals as friends, neighbors, co-workers, or family members,’’ said FBI Special Agent in Charge Richard DesLauriers. “The nation is counting on those with information to come forward.’’

The dramatic unmasking of the men who allegedly turned Boston’s annual spring sports festival Monday into a bloody scene of chaos and lost limbs, came hours after President Obama delivered an uplifting salute to the city’s resiliency in an interfaith service, which honored the three killed and the scores maimed by the blasts.


The release of the photos is the FBI’s attempt to put names to the faces, in the massive manhunt for the people who planted two primitive but powerful bombs, laced with nails and ball-bearings, that exploded 12 seconds apart on Boylston Street on Monday. Investigators have some possible names for the suspects, which they are currently checking, according to two officials familiar with the investigation.

The breakthrough in the case came after FBI specialists reviewed thousands of tips, photographs, and video footage of the race course, looking for the attackers, and found an image of a young man abandoning a backpack believed to contain a bomb. Another chilling detail that investigators found incriminating, according to the two officials: A video that showed the suspect nonchalantly striding away after the blasts, as everyone fled in panic around him.

In a 30-second video clip released Thursday by the FBI, the suspects, believed to be in their early 20s, are seen strolling single-file past Marathon spectators on Boylston Street, near the intersection with Gloucester Street. They are heading toward the finish line, walking behind a woman carrying a bouquet of yellow balloons. A time stamp appears to indicate the video was captured about 12 minutes before the bombs exploded just before 2:50 p.m.


The man the FBI has dubbed Suspect 1 is dressed in a black cap with a white Bridgestone golf logo, tan pants, and a hooded black jacket. He is shown wearing a black backpack, believed to have held the first bomb to detonate, just steps away from the finish line of the 26.2-mile race.

Suspect 2 is wearing a white baseball cap backward, dark pants, and a similar black jacket. He carries a backpack slung over one shoulder. In one photo, he appears to be smiling as he holds a cellphone to one ear. The FBI has discovered video, which it did not release, of Suspect 2 leaving his backpack on the ground at the site of the second explosion, in front of the Forum restaurant, DesLauriers said. After he dropped the pack, the man walked west on Boylston.

The bombs exploded minutes later.

DesLauriers said the FBI initially focused on one of the suspected bombers, whom they had picked out of images of the race. “Not knowing if the individual was acting alone or in concert with others, we obviously worked with extreme purpose to make that determination,’’ said DesLauriers. Through that effort, investigators discovered the other suspect.

The release of the photos followed nearly two days of silence from the FBI, during which specialists were intensely scrutinizing the images behind the scenes.

“We will continue to work on developing additional images to improve their identification value,’’ he said.

The FBI considers the men to be “armed and extremely dangerous’’ and warned the public not to try to capture the men. “If you see them, call law enforcement,’’ DesLauriers urged.


The FBI has set up a tip line, at 800-225-5324 and can take tips on its website: bostonmarathontips.fbi.gov. The FBI said it recorded record-breaking traffic on its website after releasing the images.

As they released these images, the FBI urged the public not to give weight to other photos circulating on the Internet or in the media. “The only photos that should be officially relied upon should be those you see before you today,’’ DesLauriers said.

Law enforcement officials discounted photos from the New York Post that identified two people, including a local 17-year-old, as possible suspects. They also say that the Saudi Arabian man from Revere who was injured in the blast and later questioned is not a person of interest.

The Patriots Day bombing killed 8-year-old Martin Richard, of Dorchester; 29-year-old Krystle Campbell, a Medford native; and Lingzi Lu, a Boston University graduate student from China.

Campbell’s devastated family issued a statement Thursday thanking those who tried to save Krystle’s life. “Furthermore, we are thankful for the outpouring of prayers, love, and support from our friends and families, the great community of Medford, the city of Boston, the Commonwealth of Massachusetts, the nation and the world,’’ the family said.

Lingzi Lu’s family, in a statement, said of their lost daughter: “It has always been her dream to come to America to study. While she was here, she fell in love with Boston and its people.’’

More than 170 people were wounded in the blasts, many grievously. Fifty-seven remained in Boston hospitals Thursday, including six in critical condition, down from 12 critical patients Wednesday.

At the interfaith service dedicated to the victims, President Obama delivered a moving speech designed to comfort, but not coddle.

“If they sought to intimidate us, to terrorize us, to shake us from those 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,’’ the president thundered.

It was also a thoroughly Boston-centric speech that drew on Obama’s familiarity with the city, from his time at Harvard Law School from 1988 to 1991. He mentioned city icons: the State House’s golden dome, the Boston Common and the Public Garden, the T system and Fenway Park, as well as living icons, such as the Hoyts, the father-son Marathon team honored this month with a bronze statue near the starting line in Hopkinton.

“In the words of Dick Hoyt — who’s pushed his disabled son, Rick, in 31 Boston Marathons — ‘We can’t let something like this stop us,’ ’’ Obama said. “And that’s what you’ve taught us, Boston. That’s what you’ve reminded us — to push on. To persevere. To not grow weary. To not get faint. Even when it hurts.. Even when our heart aches. We summon the strength that maybe we didn’t even know we had, and we carry on. We finish the race.’’

The president, sadly, has had plenty of practice delivering soothing words after mass killings, following attacks at Fort Hood in 2009; Tucson in 2011; and Aurora, Colo., and Newtown, Conn., in 2012. In each of those speeches, Obama spoke eloquently of the victims. His Boston remarks were unique for their vivid and damning description of the perpetrators, whom Obama called “small, stunted individuals who would destroy instead of build, and think somehow that makes them important.’’

After the service, Obama visited victims and their families at Massachusetts General Hospital, and thanked the hospital staff.

“He was extremely complimentary and I think, as he left those areas, people were thankful that he came. They were honored,’’ nurse Alice Gervasini said after the president’s visit.

Obama also met with responders and volunteers who tended to the wounded. “You’ve inspired the entire country,’’ he told them. “You’ve inspired the world. And for that, you should be profoundly proud.’’

Michelle Obama visited with patients, families, and staff at Boston Children’s and Brigham and Women’s hospitals.

Not far away, a 10-block area of the Back Bay remained closed to the public Thursday as the investigation continued, city officials said. The area included Boylston Street, where the bombings occurred, and all side streets between Newbury Street and Huntington Avenue, though those thoroughfares were open, said John Guilfoil, a spokesman for Mayor Thomas M. Menino. The area extended east to Clarendon Street, and was bounded on the west by Hereford, Dalton, and Belvidere streets.

Deb Seifert — a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms, and Explosives — said explosive specialists had processed approximately 70 percent of the scene affected by the blasts. She could not say how long the rest of the process would last. “It’s slow and methodical,’’ she said. “We have an obligation to the victims and their families to conduct a thorough investigation.’’

In Washington Thursday, Homeland Security Secretary Janet Napolitano told a congressional committee that investigators did not yet know if the bombing was a plot by domestic or foreign terrorists.

“The investigation is proceeding apace,’’ she said. “This is not an ‘NCIS’ episode. Sometimes you have to take time to properly put the chain together to identify the perpetrators, but everybody’s committed to seeing that that gets done in the right way.’’

Investigators combing the enormous crime scene earlier this week collected bomb fragments and concluded that at least one of the bombs was fashioned from a household pressure cooker.

Photos of the fragments included a battery made by Tenergy, a specialty brand typically used in radio-controlled cars

.

“It’s twisted because it’s a battery we built for a toy; to have it used this way is appalling,’’ said Benjamin Mull, an executive with the California company. “Not in a million years would I have imagined.’’