President Barack Obama is due to visit Boston in the next few hours to attend a memorial service for victims of the Boston Marathon bombing.
Two bombs were detonated near the finish line on Monday, killing three people and injuring more than 170.
The president will address an interfaith service as the search for suspects continues.
Investigators say they have found images of a potential suspect from surveillance camera footage.
Boston City Council President Stephen Murphy said a man was seen dropping off a bag at the scene on Monday.
The FBI has denied earlier widespread reports that a suspect had been arrested.
Mr Murphy said detectives had spotted the potential suspect in surveillance footage from a nearby department store, the Associated Press news agency reported.
He said he did not know whether investigators had identified the man, but added: “They may be on the verge of arresting someone and that’s good.”
Investigators have been sifting through thousands of pieces of evidence, ranging from video recorded on mobile phones to fragments of shrapnel removed from the victims.
Officials said a circuit board and battery pack – parts of a triggering mechanism – had been recovered and the lid of a pressure cooker, apparently blown off during the explosion, was found on the roof of a nearby building.
The bombs are believed to have consisted of explosives placed in 1.6-gallon (6-litre) pressure cookers, one with shards of metal and ball bearings, the other with nails.
Nobody has claimed responsibility for the bombs, which a source said had been placed in black bags and left on the ground.
The twin blasts killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, a postgraduate student from China.
Shards and shrapnel
Dozens remain in hospital following the bombs, many of them seriously injured.