The FBI released video and photos on Thursday of two suspects in the Boston Marathon bombings who were seen carrying backpacks and walking casually among spectators shortly before the blasts.

Officials hope the release of images culled from a trove of digital information will help them identify and capture the men who were described by Richard DesLauriers, special agent in charge of the FBI’s Boston division, as “armed and extremely dangerous.”

DesLauriers said at a Boston news conference that investigators identified one person of interest in the past day and that the breakthrough led to a second. He appealed for the public’s help in identifying the men.

The photos put faces on suspected perpetrators whose identity and whereabouts have been the subject of endless, often erroneous, speculation since the remotely detonated bombs killed three people and injured 176 near the race finish line Monday.

But the imagery released by the FBI , while it seemed to rule out a lone wolf, did not answer the question as to whether the bombing was a terrorist attack associated with a foreign or domestic extremist group or whether there was another, unknown agenda.

The man identified by the FBI as “Suspect 1” was carrying what appeared to be a heavy dark-colored backpack. He wore dark glasses, a black baseball hat pulled low, chino pants, a white T-shirt and a dark jacket. Des­Laur­iers said he was not captured on video dropping the backpack he was carrying.

“Suspect 2,” wearing a white baseball hat backward and dark clothing and carrying a light-colored backpack, was videotaped setting down what the FBI believes was the bomb that caused the second blast outside the Forum restaurant on Boylston Street near the finish line. The man then walked west on Boylston, but federal officials said he remained in the general area until the bomb went off.

The well-known restaurant was hosting a race-watching party at the time, and DesLauriers appealed to people who were there and who have not yet come forward to contact the FBI.

Both suspects looked like typical marathon spectators. In the snippets of released video, they appeared unhurried as they moved along the street. The video showed both men walking in single file in close proximity on Boylston Street shortly before the explosions, and DesLauriers said they appeared to investigators to be associated with each other.

“Somebody out there knows these individuals,” said DesLauriers. He said the video and photos were posted on the FBI Web site,

Officials said traffic on the Web site reached record levels after the images were released, causing a temporary slowdown.

DesLauriers said locating the suspects was the “highest priority” of investigators, but he cautioned the public not to attempt to confront or apprehend them, but to instead immediately call the FBI or police.

Officials hope the photos will be seen worldwide. In the three days since the bombing, the suspects may have left the Boston area and could well have left the country, said a law enforcement official who spoke anonymously because he was not authorized to discuss the case publicly.

The official said the FBI is also concerned about the possibility of another bombing while the two men remain at large.

Word of a break in the story caused about 120 reporters and photographers to hurry to a Sheraton hotel — not far from the marathon finish line — for the 5 p.m. FBI briefing. Bomb-sniffing dogs outside a third-floor ballroom checked all bags. Journalist and officials barely fit in the ballroom, along with two dozen television cameras.

Still images were displayed on two large black easels. One poster showed four images of the suspect with the black cap, and the other displayed four of the suspect in the white cap.

While the still and video images were slightly blurred, experts said they will help investigators determine the height, weight and body types of the suspects.

Michael Bouchard, a former assistant director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, said investigators probably weighed the consequences of releasing the images. While the disclosure could force the suspects to try to change their appearance and disappear, he said, distributing the images worldwide could lead to their identification and capture soon.

“They had to weigh the risk,” said Bouchard, who led the investigation of the D.C. sniper case and is president of Security Dynamics, a consulting firm. “Someone has seen them in their neighborhood. They have a higher likelihood of solving this quickly based on these images.”

He also said investigators are probably hoping that the pictures will jog the memories of people who might have encountered the suspects along the race course.

Earlier Thursday, President Obama eulogized the three young victims of the bombings and offered encouragement to the scores of people injured in the attacks, telling a packed cathedral in the city’s South End that “Boston’s your home town, but we claim it a little bit, too.”

“Every third Monday in April, you welcome people from all around the world to the Hub — for friendship and fellowship and healthy competition,” Obama said during an interfaith service of healing held at the soaring Cathedral of the Holy Cross. “A gathering of men and women of every race and every religion, every shape and every size, a multitude represented by all those flags that flew over the finish line.”

Bringing the crowd to its feet, the president said that the marathon held in Boston every year for well over a century exemplified the very best of the city and the nation, a resilience and determination he characterized as the ability to “finish the race.”

“Even when our heart aches, we summon the strength that maybe we didn’t even know we had. We finish the race,” Obama said. “And this time next year, on the third Monday in April, the world will return to this city and run harder than ever, and cheer louder than ever, for the 118th Boston Marathon. Bet on it.”

Obama mourned the three people killed in the bombings: Martin Richard, an 8-year-old from the Dorchester neighborhood of Boston; Lu Lingzi, a 23-year-old graduate student from China; and Krystle Campbell, a 29-year-old restaurant manager from Medford, Mass.

After the service, Obama went to nearby Cathedral High School, where he addressed hundreds of marathon volunteers and officials of the Boston Athletic Association, which organizes the marathon. The main message, Obama said, was “to say how proud the whole country is of you.”

He next went to Massachusetts General Hospital, where he met with medical staff and patients injured in the bombings. Obama’s words to the patients in their hospital beds were private. But he gave a sense when he addressed their absence at the church: “Know this: As you begin this long journey of recovery, your city is with you. Your commonwealth is with you. Your country is with you. We will all be with you as you learn to stand and walk and, yes, run again. Of that I have no doubt. You will run again.”

Finn and Horwitz reported from Washington. Julie Tate in Washington contributed to this report.