Survivors and first responders of the 2013 Boston Marathon bombing walk down Boylston Street to the marathon finish line during a Tribute Run in Boston, Massachusetts, the United States, April 19, 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)

2013 Boston Marathon bombing survivor Doug Julian (R) carries fellow survivor Lynn Crisci over the marathon finish line during a Tribute Run for survivors and first responders in Boston, Massachusetts, the United States, April 19, 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)

Survivors and first responders of the 2013 Boston Marathon bombing walk down Boylston Street to the marathon finish line during a Tribute Run in Boston, Massachusetts, the United States, April 19, 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)

2013 Boston Marathon bombing survivor Roseann Sdoia walks down Boylston Street to the marathon finish line during a Tribute Run in Boston, Massachusetts, the United States, April 19, 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)

2013 Boston Marathon bombing survivor Rebekah Gregory DiMartino (in wheelchair) attends a Tribute Run in Boston, Massachusetts, the United States, April 19, 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 along the course of 2014 Boston marathon, in Boston, Massachusetts, the United States, April 19, 2014. Some 3,500 policeman were deployed during the 2014 Boston marathon, after three people were killed and over 260 others were injured during a blast last year. (Xinhua/Yin Bogu)

Policemen guard along the course of 2014 Boston marathon, in Boston, Massachusetts, the United States, April 19, 2014. Some 3,500 policeman were deployed during the 2014 Boston marathon, after three people were killed and over 260 others were injured during a blast last year. (Xinhua/Yin Bogu)

Policemen guard along the course of 2014 Boston marathon, in Boston, Massachusetts, the United States, April 19, 2014. Some 3,500 policeman were deployed during the 2014 Boston marathon, after three people were killed and over 260 others were injured during a blast last year. (Xinhua/Yin Bogu)

Policemen guard along the course of 2014 Boston marathon, in Boston, Massachusetts, the United States, April 19, 2014. Some 3,500 policeman were deployed during the 2014 Boston marathon, after three people were killed and over 260 others were injured during a blast last year. (Xinhua/Yin Bogu)

Policemen guard along the course of 2014 Boston marathon, in Boston, Massachusetts, the United States, April 19, 2014. Some 3,500 policeman were deployed during the 2014 Boston marathon, after three people were killed and over 260 others were injured during a blast last year. (Xinhua/Yin Bogu)