The FBI has released video and photos of two "armed and extremely 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 who were seen walking together through the crowd.
One of them was seen putting down a black backpack at the site of the second blast in Monday's attack, minutes before an explosion.
"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."
He asked the public to a hotline, 1-800-CALL-FBI with tips.
DesLauriers said investigators initially zeroed in on one of the men. By examining photos and videos of the race crowd, they were able to identify a second suspect.
"They appear to be associated," he said.
The FBI released no other information about the probe, including the motive for the bombing, which killed three people and wounded 176 at the iconic race.
Other aspects of the investigation marched on. The FBI was canvassing hobby stores in the Boston area to determine whether electrical components in the bombs were bought there, NBC News learned.
The break in the case was announced hours after President Obama attended an interfaith prayer service to reassure both the injured and the city.
"You will run again," he declared at the Cathedral of the Holy Cross, less than a mile from the finish line. "Your resolve is the greatest rebuke to whoever committed this heinous act."
As Boston grasped for normal life, the city staged its first professional sports event since the blasts — a hockey match that the Bruins played against the Buffalo Sabres at TD Garden before 17,000 people.
During the national anthem, the fans gradually joined in singing, and they sent up a deafening roar over the last few bars. One YouTube video of the moment had already been viewed more than 100,000 times.
In the investigation, authorities said that they had made significant progress. However, asked whether investigators knew the identity of the people of interest in surveillance photos, Gov. Deval Patrick said, "No." He declined to elaborate.
Forensic work from the blast zone has helped authorities identify major components of the bombs.
They were housed in metal containers — at least one an everyday kitchen pressure cooker — and studded with metal, including fine nails or brads, to make the devices more lethal.
The type of battery pack used typically powers toy cars and trucks, and tens of thousands have been sold in the past year alone, which would make it difficult to trace, said Benjamin Mull, vice president of the manufacturer, Tenergy.
The FBI lashed out at news organizations after some reported Wednesday afternoon that a suspect in the case was in custody.
As of Thursday morning, 58 patients were still being treated in hospitals. That was down from 65 on Wednesday.
"In general, people are getting better, and we are happy with their progress," Dr. Peter Burke, chief of trauma at Boston Medical Center, told reporters early Thursday.
The three people killed in the attack were Lingzi Lu, a Boston University graduate student; 8-year-old Martin Richard of Boston; and 29-year-old Krystle Campbell of Medford, a Boston suburb.
A trauma surgeon said that doctors have pulled fragments as large as 2 inches, including pieces of wood, concrete and plastic, from the bodies of the injured, in addition to metal shrapnel from the bombs.