Federal investigators have released images of two men they say are sought in connection to the Boston Marathon bombings Monday.
The two men are described as suspects and, the FBI said at a 5:15 p.m. press conference held in Boston. Agents implored the public to help look.
"We know the public will play a critical role. Someone out there knows these individuals. The nation is counting on those with information to come forward and provide it to us," FBI Agent-in-Charge Richard DesLauriers said. "We consider them to be armed and extremely dangerous."
Both men are seen walking together on Boylston Street shortly before the bombings, one wearing a white hat and the other wearing a black hat. Both are carrying backpacks. The man in the black hat is walking in front of the second suspect in a white hat. The second suspect is wearing the white hat backwards.
The FBI asked anyone with any information about the suspects call (800) CALL-FBI or go to a website set up just for the public to see images of the suspects and to submit tips at bostonmarathontips.fbi.gov.
“For clarity, these [images provided] should be the only ones that the public should view,” DesLauriers said.
He noted that the suspect in the white hat is also seen in other video surveillance not released this evening planting the backpack he is carrying at the location of one of the explosions.
Within moments of the announcement, the FBI website crashed, perhaps because of a crush of visitors.
The images were released hours after President Barack Obama and first lady Michelle Obama attended an interfaith service at a Roman Catholic cathedral in Boston to remember the three people killed and more than 180 wounded in the twin blasts Monday at the marathon finish line.
The break in the investigation came just days after the attack that tore off limbs, shattered windows and raised the specter of another terrorist attack on U.S. soil.
The suspects are considered armed and extremely dangerous, DesLauriers said, and people who see them should not approach them.
"Do not take any action on your own," he warned.
Generally, law enforcement agencies release photos of suspects only as a last resort, when they need the public's help in identifying or capturing someone.
Releasing photos can be a mixed bag: It can tip off a suspect and deny police the element of surprise. It can also trigger an avalanche of tips, forcing police to waste valuable time chasing them down.
At the Cathedral of the Holy Cross earlier in the day, Obama declared to the people of Boston: "Your resolve is the greatest rebuke to whoever committed this heinous act." He spoke in almost mocking terms of those who commit such violence.
"We finish the race, and we do that because of who we are," the president said to applause. "And that's what the perpetrators of such senseless violence - these small, stunted individuals who would destroy instead of build and think somehow that makes them important - that's what they don't understand."
"We will find you," he warned those behind the attack.
Seven victims remained in critical condition. Killed were 8-year-old Martin Richard of Boston, 29-year-old restaurant manager Krystle Campbell of Medford, Mass., and Lu Lingzi, a 23-year-old Boston University graduate student from China.
The Associated Press contributed to this report.