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.