The FBI has released photos and videos of two Boston Marathon bombing suspects from just before the deadly blasts and is pleading with the public for help identifying them.
The images show two men, both wearing baseball caps, who appear to be in their mid-20s. Suspect 1 is wearing a dark cap facing forward, while Suspect 2 is wearing a backward white cap and can be seen setting down a backpack at the site of the second explosion, the FBI said.
"We believe them to be armed and very dangerous. No one should approach them," FBI Special Agent in Charge Richard DesLauriers said at a Thursday press conference. "Do not take any action on your own. If you see these men, contact the authorities."
In the images, the two men appear to be walking together through the crowd on Boylston Street, where the images were taken, DesLauriers said. He said that Suspect 2, wearing the white hat, can be seen to set down a bag in front of Forum restaurant on Boylston.
"The only one who was observed planting a device was Suspect Number 2 in the white cap," DesLauriers said. "It was shortly before the bomb blast went off, within minutes."
Law enforcement officials told NBC News that Suspect 2 was captured on unreleased portions of the video looking on as the first bomb exploded and then calmly walking away as the second bomb exploded. A still photo from another angle also showed him appearing to move away after the second blast and is being reviewed by law enforcement, officials said.
DesLauriers urged the media and the public to review, use and publicize its images to help authorities identify the suspects.
"Though it may be difficult, the nation is counting on those with information to come forward and provide it to us," he said, emphasizing that the public should use its images, and only its images, to identify the suspects. "Other photos should not be deemed credible," he said.
As of Thursday, there remained no claims of responsibility for the attacks or conclusion among authorities over whether they were acts of domestic or international terrorism, according to NBC News.
Earlier Thursday, President Barack Obama paid a visit to Boston under heavy security to offer words of reassurance to the city and a warning to those responsible for the attack: "We will find you."
Massachusetts Gov. Deval Patrick said he shared the frustration that those responsible were still at large, but he said solving the case will not "happen by magic."
"It's going to happen by doing the careful work that must be done in a thorough investigation," Patrick said. "That means going through the couple of blocks at the blast scene square inch by square inch and picking up pieces of evidence and following those trails, and that's going to take some time."
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.
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 8-year-old Martin Richard of Boston, 29-year-old Krystal Campbell of Medford, Mass., and Lu Lingzi, a Boston University graduate student from China.
NBC 4 New York's Jonathan Dienst contributed to this story.