New images of the Boston Marathon bombing suspects released by the FBI early Friday.
U.S. authorities launched a massive manhunt for two men caught on video carrying backpacks near the site of the deadly attack on the Boston Marathon, appealing to the public for any scrap of information about their identities.
The two suspects were described as armed and extremely dangerous. One was seen planting a backpack at the location of the second blast minutes before the explosion, authorities said.
To drive the search, the Federal Bureau of Investigation released several still images of the suspects. They also shared short snippets of video footage from surveillance cameras, which showed the two men walking, one just in front of the other, in the direction of the finish line of the race before the bombs exploded.
Both men are wearing baseball caps and black jackets and carrying backpacks. The first suspect is a man in sunglasses. The second suspect is a man, perhaps in his 20s, whose white baseball cap is on backward.
"Somebody out there knows these individuals as friends, neighbours, co-workers or family members," said Richard DesLauriers, the FBI agent leading the investigation into the Boston attack. "We know it may be difficult, but the nation is counting on those with information to come forward and provide it to us."
He repeatedly cautioned members of the public not to approach the men or attempt to apprehend them.
The release of the photos and video marks a major turn in the three-day-old investigation. Since the bombing on Monday, investigators have combed through thousands of photos, video clips, pieces of physical evidence and other tips.
About a day ago, they zeroed in on one of the suspects, Mr. DesLauriers said, then "worked with extreme purpose" to determine whether he was acting alone or in co-ordination with others. During that process, their interest expanded to include a second individual, he said.
Releasing photos to the public is a step law-enforcement agencies sometimes take after all other avenues are exhausted. It is a decision fraught with unintended consequences: such an appeal can alert possible suspects, prompting them to flee, or it can produce a flood of unhelpful information that ultimately squanders time and resources.
In this case, the FBI is harnessing the combined power of the mass media and the long reach of the Internet to further its goal. "In an instant, these images will be delivered directly into the hands of millions around the world," Mr. DesLauriers said at a press conference that drew hundreds of journalists, many of them broadcasting the event live.
Investigators hoped the men would be identifiable within hours of the release of the pictures and video, Reuters reported, citing a national security official who spoke on condition of anonymity. At least two other people who were initially of interest to authorities had been ruled out as suspects, the official said, including a Saudi student injured in the attacks.
Many questions remain unanswered. While authorities now believe they know which individuals carried out the attack and the type of devices used, the motive remains a mystery. No information has emerged publicly about the motivation of the would-be bombers or any connections to extremist groups, whether at home or abroad.
Mr. DesLauriers also did not say exactly what led the authorities to focus on the first suspect. Unlike the second suspect, there does not appear to be any photos or video of the first individual in the act of planting a device.
During the press conference on Thursday, Mr. DesLauriers was flanked by a phalanx of state, local and federal law-enforcement officials, including Boston's police chief and the top federal prosecutor in the city, in a move that underlined the enormous resources now engaged in the investigation.
Armed agents, police officers and soldiers in camouflage guarded the event. Prior to entering, reporters deposited their cameras and laptops so dogs could sniff them for explosives.