The Federal Bureau of Investigation (FBI) has released photographs and videos of two men investigators suspect of possible involvement in the deadly Boston Marathon bombings.
The pictures show two young men wearing dark jackets and carrying backpacks, walking behind each other on the sidewalk as marathoners run by on the street.
One is wearing a dark baseball cap, the second a white baseball cap.
The April 15 explosions near the finish line of the marathon killed at least three people and injured some 170.
FBI Special Agent Rick DesLauriers, the agent in charge in Boston, said the images were being released so that members of the public can help authorities find the suspects for questioning.
"After a very detailed analysis of photo, video, and other evidence, we are releasing photos of these two suspects," DesLauriers said.
"They are identified as Suspect 1 and Suspect 2. They appear to be associated. Suspect 1 is wearing a dark hat, Suspect 2 is wearing a white hat. Suspect 2 set down a backpack at the site of the second explosion, just in front of the Forum restaurant."
The images released by the FBI do not show the white-hatted suspect leaving behind the backpack, as described by DesLauriers.
The FBI special agent warned that the men should be considered "armed and extremely dangerous," and cautioned people to notify law enforcement instead of taking action to detain men who may be appear to be the suspects.
"No one should approach them. No one should attempt to apprehend them except law enforcement," DesLauriers said.
"Let me reiterate that caution: Do not take any action on your own."
Officials say evidence recovered from the sites of the explosions suggests that the blasts were caused by metal pressure-cooker devices, filled with explosives, nails, and steel pellets, that were concealed in backpacks or bags.
There has been no claim of responsibility for the Boston blasts, which were the most deadly attack targeting civilians in the United States since the September 11, 2001, hijacked airliner attacks.
In addition to killing three people, the Boston bombings left many of the wounded with horrific injuries, including loss of limbs.
Earlier in Boston, U.S. President Barack Obama attended an interfaith service in honor of the victims at a cathedral close to the bombing sites.
"I am here today on behalf of the American people with a simple message: everyone of us has been touched by this attack on your beloved city," Obama said.
"Everyone of us stands with you. Because after all it's our beloved city too. Boston may be your home town but we claim it too."
After his speech, Obama met at a hospital with some of the victims and also spoke with people who helped rescue the injured.
With reporting by AP and Reuters