The discovery of the surveillance video images raised hopes of a big break in the investigation, even as authorities cautioned a nation hungry for answers not to expect the case to be cracked quickly.
"Not that we are moving to imminent resolution, but we are definitely moving in the right direction," Massachusetts Gov. Deval Patrick said on his way into a memorial service where Obama and others honored the three dead and more than 180 wounded in Monday's twin blasts.
"There's a lot to follow up on, there was a tremendous amount of response to the FBI's call to the public to supply photographs and videotape that people have, and there is a tremendous amount of analysis that that requires."
At the Cathedral of the Holy Cross, 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.