The FBI released pictures and video on Thursday of two men suspected of planting the Boston marathon bombs, appealing for help to identify the pair who were carrying large backpacks.
The dramatic twist in the hunt came just after President Barack Obama vowed to the people of Boston that the "evil" bombers behind the three dead and about 180 wounded, would be brought to justice.
Investigators consider the suspects to be "armed and extremely dangerous," said Rick DesLauriers, Federal Bureau of Investigation (FBI) chief in Boston, warning that no-one should approach them.
Authorities stepped up checks on the Canadian border as they released the images. And the clamor to see the pictures was so great that the FBI's website, www.fbi.gov, crashed minutes later.
Both appeared to be young men, one dressed in a white baseball cap and the other in a black cap. But the FBI gave no details of their identities or origin, naming them only as Suspect One and Suspect Two.
Two bombs were placed around the marathon finish line on Monday, spraying nails, ball bearings and other metal fragments into the 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."
The men are seen in the video walking calmly, one a few paces behind the other, weaving between crowds on Boston's Boylston Street where the race finished.
DesLauriers said other images indicated the man in the white cap placed a bomb 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 atrocities in 2001.
The FBI says it has launched a "worldwide" hunt. An AFP reporter who crossed into Canada from Vermont state said US agents were checking all cars and trucks and asking whether drivers had been in Boston.
The FBI had already released photographs of the metal remnants of a pressure cooker believed to have been used as a bomb.
More than 100 of the wounded have left Boston hospitals and fewer than 10 of those still in hospital remain in critical condition. Some will require new operations, doctors said.
At least 12 people have lost at least one of their legs because of the blast from the bombs.
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.
The One Boston Fund (www.onefundboston.org) was set up on Tuesday by city mayor Thomas Menino and Massachusetts governor Deval Patrick.