Boston blast suspects' images released

These posters released by the FBI on Thursday, April 18, 2013, show what the FBI are calling suspect number 1, wearing a black cap, and suspect number 2, wearing a white cap, walking through the crowd in Boston on Monday, April 15, 2013, before the explosions at the Boston Marathon. AP/FBI

BOSTON (AP) — The FBI released photos and video of two suspects in the deadly Boston Marathon bombings and asked for the public's help in identifying the men. The agency's website crashed within moments Thursday.

FBI Agent Richard DesLauriers said the images are from surveillance cameras near the explosion sites shortly before Monday's blasts at the world's most famous marathon. The men are seen walking together in the crowd, and the man in the white cap is seen setting down a backpack at one site near the finish line, DesLauriers said.

"We consider them to be armed and extremely dangerous," DesLauriers said, asking the public not to approach the men. He said there is no additional danger that the FBI knows of at the moment.

Monday's blasts killed three, including a student from China, and injured more than 180.

The images came out hours after President Barack Obama promised a grieving city to hunt down whoever was responsible.

The images show two young-looking men wearing baseball caps, wearing jackets and carrying backpacks along the race route and weaving through the crowd. The planting of the backpack is not depicted in the video footage that was made public.

The FBI would not discuss the men's ethnicity.

"It would be inappropriate to comment on the ethnicity of the men because it could lead people down the wrong path potentially," said FBI agent Greg Comcowich, a spokesman for the Boston FBI office.

The information on the first suspect was developed within a day or so before its release, DesLauriers said. Agent Daniel Curtin said the FBI did not issue the photos earlier because authorities wanted to be meticulous: "It's important to get it right."

Generally, law enforcement agencies release photos of suspects only as a last resort, when they need the public's help. Releasing photos can tip off a suspect and deny police the element of surprise. It can also trigger an avalanche of tips, forcing police to waste time chasing them down.

"Each piece moves us toward justice," DesLauriers said of the latest information to emerge.

At an interfaith service honoring the victims, Obama called the perpetrators of the attack "these small, stunted individuals who would destroy instead of build."

The blasts killed 8-year-old Martin Richard, 29-year-old Krystle Campbell and Lu Lingzi, a graduate student from China. Seven victims remained in critical condition.

The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings, investigators and others close to the case said. Investigators suspect the devices were then hidden in duffel bags and left on the ground.

They exploded within 15 seconds of each other near the finish line at a high-traffic time when thousands of runners were pouring in.

Several media outlets had reported that a suspect had been identified from surveillance video taken at a Lord & Taylor department store between the sites of the bomb blasts.

The investigation will probably collect about a million hours of videotape from fixed security cameras and cellphones and cameras used by spectators, said Gene Grindstaff, a scientist at Intergraph Corp., a company that makes video analysis software used by the FBI and other law enforcement agencies.

Video and photos are being examined and enhanced by an FBI unit called the Operational Technologies Division, said Joe DiZinno, former director of the FBI lab in Virginia.

Investigators are looking at video frame by frame — a laborious process, though one aided by far more sophisticated facial recognition technology than is commercially available, forensic specialists said.

Also seen in the newly released video is a spectator walking in front of the two men, then stepping aside, holding a cluster of yellow balloons — which can be seen in the shaky moments after the first blast floating free.