Investigators have identified a suspect from security video taken before explosions at the Boston Marathon, a US law enforcement source says.
No arrests have been made, and the suspect in the video had not been identified by name, Reuters cited two US government officials as saying.
The Federal Bureau of Investigation postponed a planned news conference on Wednesday at which officials had planned to provide an update on the investigation into the Boston Marathon bombing.
FBI spokeswoman Katherine Gulotta said a bomb scare that prompted the evacuation of the city's federal court house was the reason for the postponement and it was not clear if the briefing would occur later in the day or not at all.
The identification of a possible suspect marked the most significant, publicly-disclosed break since Monday's blasts at the Boston Marathon's finish line killed three people and injured 176 others.
The bombs killed an 8-year-old boy, a 29-year-old woman and a Boston University graduate student who was a Chinese citizen. No one has claimed responsibility for the attack.
Police and the US Attorney's office in Boston said no arrest had been made, contradicting an earlier report by CNN that there had been an arrest.
Investigators have been trawling through thousands of pieces of evidence, from cellphone pictures to shrapnel shards pulled from victims' legs.
Based on shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whoever may have made bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the race watched by crowds of spectators.
Bomb scene pictures produced by the Boston Joint Terrorism Task Force and released on Tuesday showed the remains of an explosive device including twisted pieces of a metal container, wires, a battery and what appeared to be a small circuit board.
