President Obama And First Lady Attend Interfaith Service For Victims Of Boston Marathon Bombings

BOSTON, MA - APRIL 18: Former Massachusetts Governor and 2012 Republican presidential nominee Mitt Romney attends an interfaith prayer service for victims of the Boston Marathon attack titled "Healing Our City," where President Barack Obama spoke at the Cathedral of the Holy Cross on April 18, 2013 in Boston, Massachusetts. Authorities investigating the attack on the Boston Marathon have shifted their focus to locating the person who placed a black bag down and walked away just before the bombs went off. The twin bombings at the 116-year-old Boston race, which occurred near the marathon finish line, resulted in the deaths of three people and more than 170 others injured. (Photo by Spencer Platt/Getty Images)