Barack Obama has vowed to seek justice for the victims of the Boston Marathon bombing.
The US president delivered his promise at an interfaith memorial service at the Cathedral of the Holy Cross, the city's main Roman Catholic church.
He said "we will find" whoever carried out the attack that killed three people – eight year old Martin Richard; restaurant manager Krystle Campbell, 29; and Boston University graduate and Chinese citizen, Lu Lingzi.
Mr Obama – who received several standing ovations during his speech – claimed Americans would not be intimidated by the twin blasts, which also injured 176 people in a crowd of thousands at the finish line of the world-famous marathon.
"If they sought to intimidate us, to terrorise us, to shake us from those values that define us as Americans, it should be pretty clear by now that they picked the wrong city to do it to. Not here in Boston," he said.
While investigators have made no arrests, Mr Obama said of the perpetrator or perpetrators of the attack: "We will find you and you will face justice."
Boston's Mayor Tom Menino, Massachusetts Governor Deval Patrick and Cardinal Sean O'Malley also spoke to the 2000 people who filled the ornate cathedral.
Former Massachusetts governor and 2012 Republican presidential candidate Mitt Romney also attended the heavily guarded event.
The audience included scores of police officers and other first responders. Hundreds of people crowded outside the cathedral.
At the site of the bombing, more police officers stood near their squad cars, listening to the President over the radio.
Mr Obama was in solemn but defiant mood as he told the congregation: "The spirit of this city is undaunted; the spirit of this country shall remain undimmed.
"Every one of us has been touched by this attack on your beloved city. Every one of us stands with you. Because after all it's our beloved city too. Boston, you're my home. For millions of us what happened on Monday is personal."
As people exited the church after the service, crowds erupted in cheers and sang the national anthem.
Investigators are believed to have found clear images of two potential suspects carrying black bags.
The race course in central Boston on Monday was recorded by surveillance cameras and media outlets, providing investigators with significant video footage of the area before and after the two blasts.
Homeland Security Secretary Janet Napolitano confirmed the FBI was searching for people seen on a video taken near the finish line.
She said: "There is some video that has raised the question of those that the FBI would like to speak with. I wouldn't characterise them as suspects under the technical term. But we do need the public's help in locating these individuals."
Detectives believe the Boston bombs were fashioned out of pressure cookers and packed with shrapnel. They think the devices were taken to the race finish line in heavy black nylon bags. Forensics officers have recovered shards of metal, fabric, wires and a battery from the scene.
Ten victims lost limbs and emergency room doctors reported plucking nails and ball bearings from the wounded.
President Obama has declared a state of emergency in Massachusetts, a move that makes federal funding available to the state as it copes with the aftermath of the bombing.
l Double Olympic gold medal-inning long-distance runner Mo Farah has spoken of his sadness at the events in Boston, but insisted he will compete in the London Marathon on Sunday. It is due to go ahead with a heightened police presence.
Farah told BBC Radio 5 Live: "It's sad news what happened in Boston and my support goes to all the people and families."
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