After three days of poring over photos and video, investigators appealed to the public to help them identify two men now considered suspects in the Boston Marathon bombings.
The men were photographed walking down Boylston Street, one behind the other, near the finish line of Monday's race.
Suspect 1 was seen wearing a light-colored, collarless shirt underneath a dark-colored jacket and wearing a dark baseball cap.
The man identified as Suspect 2 was seen setting down a backpack at the site of the second explosion "within minutes" of the blasts that killed three people and wounded nearly 180, said Special Agent Rick DesLauriers, the head of the FBI's Boston office. He was wearing a light-colored hooded sweatshirt, a black jacket and a white baseball cap turned backward.
In particular, DesLauriers asked for help from anyone standing in front of the Forum restaurant, where the second bombing happened.
"Somebody out there knows these individuals as friends, neighbors, co-workers or family members of the suspects," DesLauriers said. "And though it may be difficult, the nation is counting on those with information to come forward and provide it to us."
People with possible information on the two men were urged to go to the FBI's website, https://bostonmarathontips.fbi.gov, or call 800-CALL-FBI (800-225-5324).
By Thursday evening, authorities already had received a "large volume of calls ... as a result of the photos," an FBI official said. The FBI's website, moreover, had been inundated with record traffic.
It wasn't immediately known then whether any of the tips had led to the suspects. DesLauriers cautioned that anyone who think they know their identities should be careful, and consider them armed and "extremely dangerous."
"No one should approach them," he said. "No one should attempt to apprehend them except law enforcement."
'They acted differently than everyone else'
Other footage, still unreleased, shows that the two suspects stayed at the scene to watch the carnage unfold, a federal law enforcement official with knowledge of the investigation told CNN's Susan Candiotti.
"When the bombs blow up, when most people are running away and victims were lying on the ground, the two suspects walk away pretty casually," said the official, who has seen the unreleased video. "They acted differently than everyone else."
While video of at least one suspect planting the bomb exists, the FBI had chosen not to release it, according to the official. One reason, according to the official, is that were the media to repeatedly show the suspects leaving the bomb, it might cause some people to overreact if they came into contact with them.
DesLauriers said intelligence had been developed on the first suspect "within the last day or so." The official who spoke with CNN said images of the second suspect were isolated Wednesday.
The investigation also turned toward the possibility that the bombs had been detonated by remote control, a federal law enforcement official briefed on the investigation said Thursday.
Investigators contacted the maker of a battery found in the debris of the blasts, said Benjamin Mull, a vice president at Tenergy Corporation. One of the firm's batteries, typically used in remote-controlled hobby cars, was found in the aftermath of the attack, connected to some wires and a piece of plastic.
In the early hours of the investigation, a law enforcement official told CNN that the bombs were probably detonated by timers. The FBI has said details of the detonating system were unknown.
While the latest clues moved the investigation forward, it is still unclear whether the attack was an act of domestic or foreign terrorism.
Bombers 'picked the wrong city,' Obama says
Thursday evening's FBI announcement capped a day in which President Barack Obama brought a mixture of reassurance and defiance to an interfaith memorial service in the city's Cathedral of the Holy Cross. Whoever planted the bombs "picked the wrong city" to attack, he said.
"Every one of us stands with you," Obama told the crowd. "Boston may be your hometown -- but we claim it, too. ... For millions of us, what happened on Monday is personal."
Addressing the still-unknown perpetrators, Obama added: "Yes, we will find you. And yes, you will face justice. We will hold you accountable." And he looked ahead to next year's race, predicting that "the world will return to this great American city to run even harder and to cheer even louder for the 118th Boston Marathon. Bet on it."
Among the crowd of about 2,000 were first lady Michelle Obama; the president's Republican challenger last November, former Massachusetts Gov. Mitt Romney; the state's current governor, Deval Patrick; and Boston Mayor Thomas Menino. During an interlude, attendees were soothed by a performance by famed cellist Yo-Yo Ma.
Menino praised each of the three bystanders who were killed in the blasts -- Martin Richard, described as a "young boy with a big heart"; Krystle Campbell, whose spirit "brought her to the marathon year after year"; and Lingzu Lu, who "came to the city in search of an education."