The FBI has released images and videos of two men wanted as key suspects in connection to the Boston marathon blasts and is asking the public for help in identifying the two men.
Richard Deslauriers, an FBI special agent, said in a press conference on Thursday that the FBI regards the suspects as "armed and extremely dangerous", adding that the agency does not believe there is any imminent danger of another attack.
The bombings that killed three people and wounded 176 began a week of security scares that rattled the United States and evoked memories of the September 11, 2001 hijacked plane attacks.
"Today we are enlisting the public's help to identify the two suspects," DesLauriers said at the news conference.
Somebody out there knows these individuals as friends, neighbours, co-workers or family members of the suspects.
Both men carried backpacks that were believed to contain the bombs.
The man identified as suspect No. 1 wore a dark baseball cap. Suspect No. 2 wore a white cap backwards and was seen setting down his backpack on the ground, DesLauriers said.
"Somebody out there knows these individuals as friends, neighbours, co-workers or family members of the suspects. Though it may be difficult, the nation is counting on those with information to come forward and provide it to us," he said.
Al Jazeera's Alan Fisher, reporting from Boston, said that with the release of the images the FBI set off one of America's biggest ever manhunts.
"These two men are now the most wanted men in America and it is understood that investigators have in the last hour or so already been passed a number of names, some of which are recurring. So those leads will obviously be checked out."
Investigators hoped the men would be identifiable within hours of the release of the pictures and video, a national security official said, speaking on condition of anonymity.
Investigators were looking at the men for some period of time before deciding to make the videos public, and they had extensive video and still pictures to justify the FBI decision to label the two men as suspects, the official said.
At least one other person of interest who featured in crime scene pictures had been ruled out as a suspect.
Also ruled out earlier in the week was a Saudi student who was injured in the attacks, the official said.