A National Guardsman stands guard in front of the scene of the explosion in Boston, the United States, April 15, 2013. The two explosions that rocked the Boston Marathon on Monday has killed three people and injured at least 138, officials and media outlets said. (Xinhua/Marcus DiPaola) 

National guardsmen and police stand guard in front of the scene of the explosion in Boston, the United States, April 15, 2013. The two explosions that rocked the Boston Marathon on Monday has killed three people and injured at least 138, officials and media outlets said. (Xinhua/Marcus DiPaola)

Policemen talk in front of the scene of the explosion which killed three people in Boston, the United States, April 15, 2013. The two explosions that rocked the Boston Marathon on Monday has killed three people and injured at least 138, officials and media outlets said. (Xinhua/Marcus DiPaola)

National guardsmen and police stand guard in front of the scene of the explosion in Boston, the United States, April 15, 2013. The two explosions that rocked the Boston Marathon on Monday has killed three people and injured at least 138, officials and media outlets said. (Xinhua/Marcus DiPaola)

Boston EMS carries materials in front of the scene of the explosion in Boston, the United States, April 15, 2013. The two explosions that rocked the Boston Marathon on Monday has killed three people and injured at least 138, officials and media outlets said. (Xinhua/Marcus DiPaola)

An ambulance and armored car sit at the scene of the explosion which killed three people in Boston, the United States, April 15, 2013. The two explosions that rocked the Boston Marathon on Monday has killed three people and injured at least 138, officials and media outlets said. (Xinhua/Marcus DiPaola)