The FBI on Thursday released pictures and video of two men suspected of involvement in the deadly bomb attacks at the Boston marathon, appealing to the public to help identify them.
The dramatic turn came only hours after President Barack Obama vowed to the people of Boston that the "evil" bombers who attacked Monday's race, killing three and wounding more than 180, would be found and brought to justice.
Investigators consider the two men to be "armed and extremely dangerous," said Rick DesLauriers, Federal Bureau of Investigation (FBI) chief in Boston.
The FBI's website, www.fbi.gov, crashed in the minutes after the pictures and video footage were released.
The FBI said it had no details of the identities or origin of the two men, who were only named as Suspect One and Suspect Two. Both appeared to be young men, one dressed in a white baseball cap and one in a black cap.
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Two bombs were placed at the finish line of the marathon, spraying nails, ball bearings and other metal fragments into the huge crowds, many of whom suffered horrific injuries.
"Identifying and locating those responsible is now our highest priority," DesLauriers said.
"The images from Monday are indelible and the horror of that day will remain with us forever," DesLauriers told reporters.
The men are seen in still images and video released by the FBI walking along Boston's Boylston Street amid crowds watching the race finale. Both had large backpacks.
The FBI said the images indicated that the man in the white cap had placed one of the two bombs outside a restaurant in the street in the minutes before the blasts tore through the crowds.
At a special service at Boston's Cathedral of the Holy Cross, Obama vowed: "Yes, we will find you, and yes, you will face justice."
"We will find you, we will hold you accountable," he told a congregation of 2,000 including relatives of the dead, survivors of the blasts, rescuers and city leaders.
"If they sought to intimidate us, to terrorize us," Obama said, then "it should be pretty clear by now that they picked the wrong city to do it."
Obama met the family of Krystle Campbell, a 29-year-old who was killed, at the cathedral before the service, and later went to Massachusetts General Hospital to talk to some of the wounded.
Americans had seen "the face of evil" in the attacks, the president told the service where Muslim, Jewish, Roman Catholic, Presbyterian and Greek Orthodox religious leaders spoke.
No claim of responsibility has been made in connection with the worst terror attack in the United States since the September 11, 2001 atrocities.
The FBI says it has launched a "worldwide" hunt. But the FBI and political leaders have appealed for patience over the pace of the investigation.
The FBI had already released photographs of the mangled metal remnants of a pressure cooker believed to have been used for one of the bombs. The lid of one pressure cooker was found on the roof of a hotel near the marathon finish line.
More than 100 of the injured have left Boston hospitals and fewer than 10 of those still in hospital remain in critical condition. Some will require new operations.
At least 12 people have lost at least one of their legs because of the blast from the bombs, which fired the metal fragments at low level.
Boston has held emotional tributes to the dead -- eight-year-old Martin Richard, Boston University graduate student Lu Lingzi of China and Campbell, a restaurant manager.
Obama paid tribute to all three at the service.
Doctors at Boston Medical Center said a second Chinese student caught in the blast had come out of a coma and was improving. The girl's family was expected in Boston soon.
Meanwhile a special fund set up for victims has raised more than $10 million in less than two days, local media reports said.
