The FBI has released photos of two suspects it wants to identify as part of the investigation into Monday's Boston Marathon bombings.
CCTV captured the two men, one wearing a dark-coloured baseball cap and the other a white cap, near the scene.
FBI Agent Richard DesLauriers warned members of the public not to approach the two men.
Three people were killed and more than 170 wounded when the two bombs exploded near the finish line of the race.
Police are carrying out a major operation in Watertown, six miles (9km) from Boston, after witnesses reported hearing explosions and gunfire in the town.
A police officer was shot dead late on Thursday at the Massachusetts Institute of Technology (MIT) campus in between Watertown and Boston.
At least one man has been arrested in Watertown, and the FBI has told US broadcaster CNN it is trying to establish whether there is a connection.
Some 14 victims, including three children, remain in hospital in a critical condition after Monday's bomb attack.
Mr DesLauriers told a news conference on Thursday that the two suspects were "armed and extremely dangerous".
He said footage showed the suspect in the white hat putting his backpack down at the site of the second explosion just in front of the Forum restaurant.
"We strongly encourage those who were at the Forum restaurant but haven't contacted us yet to do so," he said.
The FBI published the images on its website along with a 30-second video showing the men walking in single file a few paces apart along Boylston Street in central Boston.
Mr DesLauriers added: "Somebody out there knows these individuals as friends, neighbours, co-workers, or family members of the suspects. Though it may be difficult, the nation is counting on those with information to come forward."
At least one of the Boston bombs was made from a pressure cooker packed with explosives, nails and ball bearings, investigators have told US media. The devices were placed in black bags and left on the ground.
Earlier, President Barack Obama told an interfaith ceremony at the Cathedral of the Holy Cross in Boston that everyone had been touched by the attack on their city.
"The spirit of this city is undaunted; the spirit of this country shall remain undimmed," he told the congregation, which included city leaders, residents and victims at the prayer service.
"Every one of us has been touched by this attack on your beloved city. Every one of us stands with you. Because after all it's our beloved city too.
"They sought to intimidate us, to terrorise us. It should be pretty clear by now that they picked the wrong city to do it."
He was applauded as he warned the perpetrators of the attacks they would be brought to justice.
Hundreds of people stood outside the cathedral during the service, while police at the bomb site listened to the president's speech on the radios in their patrol cars.
Investigators have been going through thousands of images taken around the time of the attacks from security cameras, the media and people at the scene.
The twin blasts killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, a postgraduate student from China.