FBI Agent Richard DesLauriers says the photos came from surveillance cameras near the explosion sites and that he's asking the public to help identify the two men.
"They appear to be associated," DesLauriers told a press conference late Thursday afternoon.
The photos show one man, identified as suspect one, as wearing a dark baseball cap. Suspect two is wearing a white hat and is seen setting down a backpack at the site of the second explosion just in front of the Forum restaurant, DesLauriers said.
He added that the backpack was set down minutes before the blast that left three people dead, wounded more than 170.
- Read the timeline of the Boston Marathon bombing
- Learn the history of the pressure cooker bomb
The suspects appear to be walking together through the marathon crowd on Boylston Street in the direction of the finish line, he said.
"We consider them to be armed and extremely dangerous. No one should approach them. No one should attempt to apprehend them except law enforcement," he said.
Homeland security committee chair Michael McCaul said it's possible the suspects could head to Canada.
"The Canadian border is not very far from Boston and the idea of them getting in a car and driving across the border into Canada is another flight risk that I would be very concerned about," he told CNN.
The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings, investigators and others close to the case said. Investigators suspect the devices were then hidden in black duffel bags and left on the ground.
They exploded within 15 seconds of each other near the finish line at a high-traffic time when thousands of runners were pouring in.
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Several media outlets had reported that a suspect had been identified from surveillance video taken at a Lord & Taylor department store between the sites of the bomb blasts.
7 bombing victims still critical
Seven bombing victims remained in critical condition.
Dr. Peter Burke, chief of trauma surgery at Boston Medical Center, said Thursday that one of the youngest victims, a 5-year-old boy is getting better and "is going to be OK." A blast can often compress a child's chest, bruising the lungs and heart, he said, adding he is pleased with the boy's progress.
Dozens of victims have been released from hospitals, and officials at three hospitals that treated some of the most seriously injured said they expected all their remaining patients to survive.
The blasts killed:
- Martin Richard, 8, of Boston.
- Krystle Campbell, 29, of Medford, Mass.
- Lu Lingzi, a Boston University graduate student from China.