The FBI released photos and a brief videotape Thursday of two men it described as "Suspect No. 1 and Suspect No. 2" in the bombing Monday at the Boston Marathon that killed three and injured nearly 180, and appealed to the public to help identify them.
The men, walking single file on Boylston Street toward the marathon's finish line, were seen in photos and a 30-second video carrying black backpacks as they walked briskly along the marathon route. Both wore dark jackets. One wore a white baseball cap, turned backward, and the other wore a black one.
Richard DesLauriers, special agent in charge of the FBI's Boston field office, said during a press briefing that the photos and video came from surveillance cameras near the explosion sites.
One of the unidentified men is believed to have planted the bombs near the finish line of the race, he said, and both men are considered armed and dangerous.
The images were released hours after President Obama and first lady Michelle Obama attended an interfaith service at a Roman Catholic cathedral in Boston to remember the victims.
Mr. DesLauriers said the man labeled as Suspect No. 2, who was wearing the white cap, was seen leaving his bag "within minutes" at the site of the second explosion. He said Suspect No. 2 also was seen walking west on Boylston Street after the bomb detonated.
"We initially developed a single person of interest," not knowing whether the man was acting alone or with others, Mr. DesLauriers said. He said the FBI, working with state and local law enforcement authorities, later determined that at least two men were involved in the attack.
"With the media's help, we know the public will create a critical role in locating these suspects," he said. "The nation is counting on those with information to come forward."
He said anyone with information should call 1-800-CALL-FBI or go to the bureau's website, FBI.gov. Shortly after asking for the public's help, the FBI website crashed from the crush of visitors.
Mr. DesLauriers said no other suspects have been identified in the case and the FBI was focused on identifying and finding the two men captured in the photos and video.
Federal, state and local law enforcement authorities have been meticulously reviewing videotape from retail stores and restaurants near the bombing site as well as videos and photographs from citizens, news organizations and other sources over the past three days.
Agents also have scoured the sites of the explosions to locate and possibly reconstruct the bombs. They found pieces of an electronic circuit board possibly indicating a timer was used in the detonation of at least one of the bombs.
Investigators think the bombs were assembled from pressure cookers packed with black powder as well as nails and pellets, similar to explosive devices used in attacks worldwide. They also found the lid of one pressure cooker on the rooftop of an adjoining building. Pressure cookers filled with explosives amplify the damage from the explosions by briefly constraining the blast.
Much of the evidence has been shipped for further examination by forensics specialists at the FBI lab in Quantico, Va.
A videotape from the Lord & Taylor department store near the marathon finish line purports to show a young man dropping a heavy black bag at the site of the second bombing.
The blasts occurred shortly before 3 p.m. Monday near the finish line on Boylston Street, transforming a prized Boston tradition into a gruesome scene. Doctors said they had to amputate limbs and pull metal shards from the injured.
The victims who died have been identified as Martin Richard, 8; Krystle Campbell, 29; and Chinese citizen Lu Lingzi, who had been studying mathematics and statistics as a graduate student at Boston University.
Many of the injured remained in hospitals, seven with critical injuries.
Earlier Thursday, Homeland Security Secretary Janet A. Napolitano testified before the House Homeland Security Committee that it was unclear whether those who planted the bombs at the marathon were foreign or domestic terrorists.
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