Barack Obama, the US president, has said masterminds of blasts that killed three and injured 176 people in the city of Boston, Massachusetts, will be held "accountable", adding that the US will not "cower in fear".
Speaking at the interfaith service dedicated to those gravely wounded or killed in Monday's explosions, Obama also called the attackers "small, stunted individuals" who were out to cause terror.
"Yes, we'll find you and, yes, you will find justice and we'll hold you accountable," he said on Thursday.
"They [attackers] picked the wrong city to do it. Not here in Boston, not here in Boston," he said as the congregation erupted in rounds of applause.
Al Jazeera's Alan Fisher, reporting from Boston, said the president hit the right tone, citing comments on social media.
Our correspondent said Obama had a strong connection to Boston as he and wife Michelle, who accompanied him to the service, went to Harvard, the city's and America's best known university.
"He knows the areas. He said that he and Michelle had walked through many of the streets; many of the neighbourhoods."
Obama said Boston would stand up again and that its "resolve is the greatest rebuke to whoever committed" the deadly attack.
Obama said what was meant to be a "celebration became a tragedy", referring to the two blasts.
"Today we come together and pray and mourn and measure our loss," he said.
"On behalf of the American people, I've a simple message: everyone of us has been touched … everyone of us stands with you because it [Boston] is our city too … it's one of America's iconic city and one of the world's greatest city."
Earlier, Thomas M. Menino, the mayor of Boston, said: "I have never loved its people more than I do today. We are one Boston. Nothing can tear down the resilience of this city."
Obama finds himself trying to heal emotional wounds barely four months after he offered solace to the families of 20 school children and six educators killed in a shooting in Newtown, Connecticut.
His trip to Boston came a day after the Senate failed to muster enough votes to pass expanded background checks for firearms purchases. The president has been pushing for tighter gun controls in the wake of the Newtown shooting.
Meanwhile, Janet Napolitano, homeland security secretary, said the Federal Bureau of Investigation (FBI) wanted to speak to two men seen in at least one video from the Boston Marathon, but she stopped short of calling them suspects.
Without providing details of the men's appearance or what the video shows, Napolitano told the House Homeland Security Committee on Thursday that "there is some video that raised the question" of two men the FBI would like to interview.
Napolitano said it was still unclear whether the bombs were the work of foreign or domestic terrorists. She said the investigation was continuing "apace".