Boston - US President Barack Obama heads to Boston on Thursday to mourn victims of the deadly marathon attacks, as investigators study images of a suspect who may have planted the bombs.
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.
But the images from a surveillance camera and still pictures could mark a breakthrough ahead of the visit to Boston by Obama and his wife Michelle.
Obama, who will speak at a special inter-faith morning service for the victims at the Cathedral of the Holy Cross, has condemned the attack as "an act of terror" and vowed that the attackers "will feel the full weight of justice".
Despite increased security across the city, Boston remained on edge.
The federal courthouse and part of one hospital where victims are being treated were briefly evacuated on Wednesday, while conflicting reports of an arrest brought a stern rebuke from the Federal Bureau of Investigation.
But a law enforcement official said "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 running away while other people slumped to the ground during the blast.
With no claim of responsibility made for the attack, the FBI said it has launched a "worldwide" hunt.
Massachusetts Governor Deval Patrick appealed for patience over the pace of the investigation.
"They are making progress. But it's going to take time," Patrick said on CNN. "Every hour, we're closer."
Nails handed over to police
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.
Doctors at hospitals where the injured were taken say ball bearings and nails taken from patients were being used in the investigation.
George Velmahos, Massachusetts General Hospital's chief of trauma surgery, said the metal was being handed over to police. He said 12 nails were taken from inside one patient alone.
Peter Burke, chief trauma surgeon at Boston Medical Centre, said some of the nails were about 5cm long.
US authorities have thrown virtually every investigative agency into the hunt for the bombers.
About 100 of the injured have left Boston hospitals, but about 10 remain in critical condition and will require new operations.
Boston's federal courthouse and part of the Brigham and Women's Hospital, where many victims were taken, were evacuated on Wednesday.
Hundreds of people went to the courthouse after reports that a suspect had been arrested for the attacks.
But the reports were denied by Boston police, and the FBI criticised what it called a number of "inaccurate" press reports since the attacks.
"Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting," it said in a statement.
Boston has held emotional tributes to the dead, who include 8-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. Thousands of tributes to Lu were posted on Chinese websites.