Boston - Investigators have an image of a potential suspect in the deadly Boston Marathon bombing but do not know his name and have not questioned him, a law enforcement official said on Wednesday.
The official said investigators made the discovery while looking over photos and video of Monday's twin blasts, which killed three people and wounded more than 170.
The official was not authorised to discuss the case publicly and spoke to The Associated Press on condition of anonymity.
Police and reporters converged on the federal courthouse in a jittery city amid reports of a breakthrough in the investigation and conflicting information on whether a suspect was in custody.
Several media outlets reported that a suspect had been identified from surveillance video taken at a Lord & Taylor department store between the sites of the two bomb blasts.
President Barack Obama called the attack on the world's most famous marathon an act of terrorism. Obama planned to attend an interfaith service on Thursday in the victims' honour in Boston.
A bomb threat forced the evacuation of the courthouse in midafternoon, the US marshals service said, but workers were allowed back into the courthouse a short time later.
A law enforcement official briefed on the investigation told The Associated Press earlier in the day that a suspect was in custody.
But the FBI and the US attorney's office in Boston said no arrests had been made.
Law enforcement agencies had pleaded for the public to come forward with photos, videos or any information that might help them solve the twin bombings near the race's finish line.
The bombs exploded 10 or more seconds apart just next to the race course, tearing off victims' limbs and spattering streets with blood. The blasts went off minutes after the four-hour mark of the race, a high-traffic time when thousands of runners pour toward the finish.
The bombs involved kitchen pressure cookers packed with explosives, nails and other lethal shrapnel. But the FBI said nobody had claimed responsibility.