Richard DesLauriers, FBI special agent in charge, said Thursday the two suspects -- identified as "Suspect 1" and "Suspect 2" -- "appear to be associated" and were photographed walking side-by-side near the intersection of Boylston Street and Gloucester Street near the marathon's course.
DesLaurier said video shows one of the suspects dropping a package along the marathon route.
"We consider them to be armed and extremely dangerous," DesLauriers said. "No one should approach them."
DesLauriers said "somebody out there knows these individuals" and urged the public to report any information on their identity online at bostonmarathontips.fbi.gov or by calling toll-free at 800-CALLFBI.
During an interfaith service earlier in the day, President Barack Obama said resolve is the "greatest rebuke" to those behind the Boston Marathon bombings.
The country will be with Boston and racers as they stand, walk and run again, said Obama, the final speaker at the "Healing Our City: An Interfaith Service" at the Cathedral of the Holy Cross.
Faith leaders offered prayers from their holy books, praised first responders and others who provided assistance and rendered aid and honored the three people who died and the more than 170 who were injured when two bombs exploded near the finish line of the Boston Marathon Monday.
"I have no doubt that you will run again. That's what the people of Boston are made of," Obama said to rousing applause. "Your resolve is the greatest rebuke to whoever committed this heinous act."
"If they sought to intimate us, to terrorize us, to shake us from our values," Obama said, it should be pretty clear right now that they picked the wrong city to do it. Not here in Boston."
All of America has been touched by "this attack on your beloved city, every one of us stands with you," Obama said, because "it's our beloved city, too."
The ecumenical event that drew together leaders of many faiths as well as state and local leaders, including former Massachusetts Gov. Mitt Romney, who challenged Obama in the 2012 election.
To the perpetrators, Obama said: "[We] will find you and yes, you will face justice," an emotional Obama said in measured tones. "We will find you. We will hold you accountable. But more than that, our fidelity to our way of life ... will only grow stronger."
The perpetrators -- "small, stunted individuals" -- Obama said, don't understand that Americans' faith in each other and love for each other and the country "is our power."
"That's why a bomb can't beat us," he said. "We carry on. We race. We strive. We build and we work and we love and we raise our kids to do the same and we come together to celebrate life."
"You showed us, Boston that in the face of evil, [we] will lift up what's good," Obama said. "In face of cruelty, we will choose compassion. In face of those who would visit death upon innocents, we will choose to save and to comfort and to heal. We'll choose friendship. We'll choose love."
And on the third Monday of April 2014, "the world will return to this great American city to run harder than ever, to cheer even louder for the 118th Boston Marathon," Obama said to more applause and another standing ovation.
"Bet on it."
Massachusetts Gov. Deval L. Patrick vowed there would be accountability "without vengeance" and "vigilance without fear."
Long after the world moves on, Patrick said he hoped "the grace this tragedy exposed is the best of who we are."
"On Boston's streets Monday afternoon ... we saw souls murdered but also lives saved," said Bassar Wedaddy, chairman of the area's interfaith council.
Boston Mayor Thomas Menino said "no adversity, no challenge" can counter the resiliency of the city's people.
"Nothing can defeat the heart of the city," Menino said. "Nothing will take us down because we take care of one another."
Boston will triumph over the Monday's "hateful act" and will push forward, he said.
"This is Boston," he said, "a city with courage, compassion and strength that knows no bounds."
Obama signed an emergency declaration Wednesday for Massachusetts and ordered federal aid to help the local response to the bombings.
The bombings killed Martin Richard, 8, of Dorchester, Mass., Krystle Campbell, 29, of Arlington, Mass., and Boston University graduate student Lingzi Lu, 23, a native of Shenyang, China.