The FBI has released video and photos of two "armed and dangerous" suspects in the Boston Marathon bombing and appealed for the public's help in identifying them.

The images show two men in baseball caps. One of them was seen putting down a black backpack at the site of the second blast in Monday's attack. Both were seen walking together through the crowd.

"Somebody out there knows these individuals…though it may be difficult, the nation is counting on those with information to come forward and provide it to us," Boston FBI Special Agent in Charge Richard DesLauriers said at a Thursday briefing.

"We consider them to be armed and extremely dangerous," DesLauriers said of the men. "No one should approach them...If you see these men, contact law enforcement."

The FBI was also canvassing hobby stores in the Boston area to determine whether electrical components in the bombs were bought there, NBC News learned.

At an interfaith prayer service, President Barack Obama reassured both the injured and the city.