Policemen stand under a screen near the site where explosions occured during the 2013 Boston marathon, in Boston, Massachusetts, the United States, April 15, 2014. Twin bombs placed in backpacks near the finish line of Boston Marathon killed three people and injured more than 260 on April 15, 2013. (Xinhua/Yin Bogu)

Policemen guard the site where explosions occured during the 2013 Boston marathon, in Boston, Massachusetts, the United States, April 15, 2014. Twin bombs placed in backpacks near the finish line of Boston Marathon killed three people and injured more than 260 on April 15, 2013. (Xinhua/Yin Bogu)

Policemen pass by a screen near the site where explosions occured during the 2013 Boston marathon, in Boston, Massachusetts, the United States, April 15, 2014. Twin bombs placed in backpacks near the finish line of Boston Marathon killed three people and injured more than 260 on April 15, 2013. (Xinhua/Yin Bogu)

Policemen pass by the site where explosions occured during the 2013 Boston marathon, in Boston, Massachusetts, the United States, April 15, 2014. Twin bombs placed in backpacks near the finish line of Boston Marathon killed three people and injured more than 260 on April 15, 2013. (Xinhua/Yin Bogu)

A bunch of flowers can be seen at the site where explosions occured during the 2013 Boston marathon, in Boston, Massachusetts, the United States, April 15, 2014. Twin bombs placed in backpacks near the finish line of Boston Marathon killed three people and injured more than 260 on April 15, 2013. (Xinhua/Yin Bogu)