Investigators have revealed they have an image of a potential suspect taken from security footage before the deadly Boston Marathon bombings.
The picture is believed to have come from CCTV video taken before the twin attacks, in what could be the biggest breakthrough in the case.
It apparently shows a man carrying and possibly dropping a black bag at the second bomb scene on Boylston Street, outside the Forum restaurant, and then walking away.
The surveillance video is believed to have been taken at the Lord and Taylor department store between the sites of the two blasts, which were seconds and about 100 metres apart on the same road.
Mr Obama and First Lady Michelle will attend an interfaith service at the Cathedral of the Holy Cross later today in the city.
Former Massachusetts Governor and Republican presidential nominee Mitt Romney is also expected to attend.
It comes as police said no-one has been arrested over Monday's bombings, despite reports in the US media that a suspect was in custody.
The Associated Press earlier claimed a law enforcement official had said a person was detained and was set to be taken to the local federal court.
Other media outlets also reported an arrest. But the Federal Bureau of Investigation, the US attorney's office in Boston and the police disputed that.
"Despite reports to the contrary there has not been an arrest in the Marathon attack," the Boston police department said on Twitter.
The FBI also said there had been "no arrests made," urging the media to "exercise caution".
Boston City Council president Stephen Murphy, who was 50 yards from one of the explosions, told Sky News the investigation could be long and drawn out.
He said: "There were tens of thousands of people out there on Boylston Street and so much movement and so much going on and so much to look at, and everybody's emailing, texting tips, to law enforcement.
"People are poring over that."
A sequence of photographs taken by Boston businessman Ben Thorndike show a man in burned clothes fleeing from the scene immediately after the two blasts.
"I was struck by this individual,” he told ABC News.
"Everyone else in the photo is on the ground, bent over, holding their ears, appearing to be in shock, immobile, but this one individual is exactly the opposite."
The explosive devices were made from pressure cookers hidden inside duffel bags packed with nails, shards of metal and ball bearings, placed on the ground near the finish line of the race.
Three people were killed and more than 170 others were injured in the twin blasts on Monday.
Those who died were eight-year-old Martin Richard, who was watching with his family at the finish line; restaurant manager Krystle Campbell, 29, and Chinese graduate student Lu Lingzi, who was studying at Boston University.
Scores of victims are still being treated in hospital, many with serious injuries. Seventeen people, one as young as five, remain in a critical condition.