U.S. President Barack Obama says the nation's resolve is the "greatest rebuke" to the terrorists responsible for two deadly explosions at Monday's Boston Marathon.
The president told those attending an interfaith memorial service at the Cathedral for the Holy Cross in Boston, Massachusetts, the city has reminded Americans to "push on" and "persevere".
"We may be momentarily knocked off our feet, but we will pick ourselves up, we will keep going, we will finish the race," he said.
He asked God to hold close those who lost their loved ones in the tragedy and warned those responsible they would be brought to justice.
American cellist Yo-Yo Ma filled the cathedral with a beautiful, somber melody. Boston Cardinal Sean O'Malley and Massachusetts Governor Deval Patrick also spoke at the service.
A White House spokesman says the president will meet with families of those killed or wounded, along with some of the first responders to the tragedy, while in Boston.
Law enforcement officials are still working to identify those responsible for the bombs from video and photo evidence.
Homeland Security Secretary Janet Napolitano told the House Homeland Security Committee the FBI is looking for people of interest seen in video from the Boston Marathon, without going as far as calling them suspects.
Media reports say investigators are focused on two possible suspects in Monday's explosions that left three people dead and wounded more than 170.
They also report video captured a man seen dropping off a bag and then walking away from the site of the second bombing. The images found on surveillance video from a department store near the finish line have not been made public.
On Wednesday, the FBI, federal prosecutors and Boston police denied widespread media reports a suspect in the case had been arrested.
Two blasts seconds apart near the finish line of Monday's marathon killed an 8-year-old boy, a restaurant catering manager and a graduate student at Boston University.
U.S. President Barack Obama speaks at an interfaith memorial service for the victims of the bombing at the Boston Marathon at the Cathedral of the Holy Cross in Boston, Massachusetts, April 18, 2013.
People attend the interfaith memorial service at the Cathedral of the Holy Cross for the victims of the Boston Marathon bombing in Boston, April 18, 2013.
Nursing students Katie Robinson, left, and Megan Beach listen to a broadcast on their phones outside an interfaith service attended by President Barack Obama at the Cathedral of the Holy Cross, held in the wake of Boston Marathon explosions.
U.S. President Barack Obama and First Lady Michelle Obama arrive for an inter-faith memorial service for the victims of the bombing at the Boston Marathon in Boston, April 18, 2013.
A memento of flowers in a running shoe rests at a makeshift memorial in Boston's Back Bay neighborhood, April 18, 2013, a few blocks from the finish line of the Boston Marathon.
Homeland Security Secretary Janet Napolitano testifies on Capitol Hill in Washington, April 18, 2013, before the House Homeland Security Committee. Napolitano says the FBI wants to speak with two men seen in at least one video from the Boston Marathon, but she says she isn't calling them suspects.
A line of investigators is form as they enter a building adjacent to one of the blast sites near the Boston Marathon finish line, in Boston, April 18, 2013.
Investigators inspect the area between the two blast sites near the Boston Marathon finish line, Thursday, April 18, 2013, in Boston. Boston remained under a heavy security presence, with scores of National Guard troops gathering among armored Humvees in
Local residents attend a candlelight vigil in the Dorchester neighborhood of Boston, Massachusetts, April 16, 2013, where eight-year-old victim Martin Richard lived.
Investigators comb through the post finish line area of the Boston Marathon at Boylston Street, two days after two bombs exploded just before the finish line, April 17, 2013, in Boston.
A worker returns a bag containing a runner's personal effects near the finish line of the Boston Marathon, after explosions killed three and injured more than 140 in Boston, April 16, 2013.