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 dead and the more than 180 wounded in the twin blasts Monday at the finish line of the 26.2-mile race.
The FBI video is a compilation of segments, altogether about 30 seconds long. The planting of the backpack, as described by authorities, was not part of the footage made public.
The man in the dark hat was dubbed Suspect 1 and appeared to be wearing sunglasses. The other, in the white hat, was labeled Suspect 2. Both appeared to be wearing dark jackets. The FBI did not comment on the men's height, weight or age range and would not discuss their ethnicity.
"It would be inappropriate to comment on the ethnicity of the men because it could lead people down the wrong path potentially," said FBI agent Greg Comcowich, a spokesman for the Boston FBI office.
While authorities said the information on the men began coming together over the previous day or so, agent Daniel Curtin said the FBI did not release the photos earlier because "it's important to get it right."
Distribution of the images brought both encouragement and unease to some Bostonians.
Jennifer Lauro of Topsfield, worried that the photos might breed unease and suspicion.
"It just looks like a college kid, so I think that's going to make people feel vulnerable," she said. "Because it could be anybody. It looks like any kid from Boston College or Boston University or any other school."
Judy and Marc Ehrlich watched the marathon from a spot between miles 25 and 26 on Monday and felt the ground shake when the bombs exploded. The couple said it was creepy to see images of the suspects who were there at the same time, walking around. But they were comforted that the FBI had come up with suspects.