President Barack Obama headed to Boston on Thursday to mourn victims of the deadly marathon attacks, as investigators study surveillance footage they are hoping will lead them to identify those responsible.
No arrests have been made in connection with Monday's twin bombings near the finish line of the race, which sent metal fragments and nails into a crowd of thousands of runners and spectators, killing three people and wounding 180.
Obama, who will speak at a special inter-faith memorial service for the, has condemned the attack as "an act of terror" and vowed that the attackers "will feel the full weight of justice."
While authorities have yet to identify the perpetrators, a law enforcement official told AFP that, "there is an image of a person, of a potential suspect." Investigators were working "to locate and identify that individual," the official added.
The Los Angeles Times and CNN were reporting that investigators were looking at two possible suspects based on the images, but that the men were as yet unidentified.
Media reports said the images may have captured at least one suspect dropping a backpack that may have held the bomb. Reports also indicated the pictures showed the suspect fleeing the scene while just prior to the blasts.
With no claim of responsibility made for the attack, the FBI said it has launched a "worldwide" hunt.
"They are making progress. But it's going to take time," Massachusetts Governor Deval Patrick told CNN. "Every hour, we're closer."
The FBI released photographs of the mangled metal remnants of a pressure cooker believed to have been used for one of the bombs, which sprayed nails, ball bearings and metal pellets into the crowds.
The lid of one pressure cooker was found on the roof of a nearby hotel.
Boston has held emotional tributes to the dead, who include eight-year-old Martin Richard, Boston University graduate student Lu Lingzi of China and restaurant manager Krystle Campbell.
About 1,000 people attended a candlelight vigil in a park near the boy's home on Tuesday night and hundreds went to other events in the city and at the university.