People take part in a ceremony commemorating the first anniversary of the Boston Marathon bombings in Boston, the United States, April 15, 2014. Thousands of people Tuesday came together at Boylston Street in central Boston to mark the first anniversary of the Marathon bombings with silent tribute, church bells tolling and a flag-raising ceremony. (Xinhua/Yin Bogu)

Police block the streets during a ceremony commemorating the first anniversary of the Boston Marathon bombings in Boston, the United States, April 15, 2014. Thousands of people Tuesday came together at Boylston Street in central Boston to mark the first anniversary of the Marathon bombings with silent tribute, church bells tolling and a flag-raising ceremony. (Xinhua/Yin Bogu)

People walk past a board with the names of the Boston Marathon bombings victims in Boston, the United States, April 15, 2014. Thousands of people Tuesday came together at Boylston Street in central Boston to mark the first anniversary of the Marathon bombings with silent tribute, church bells tolling and a flag-raising ceremony. (Xinhua/Yin Bogu)

A policeman stands guard during a ceremony commemorating the first anniversary of the Boston Marathon bombings in Boston, the United States, April 15, 2014. Thousands of people Tuesday came together at Boylston Street in central Boston to mark the first anniversary of the Marathon bombings with silent tribute, church bells tolling and a flag-raising ceremony. (Xinhua/Yin Bogu)