FILE - Medical workers aid injured people following an explosion at the finish line of the 2013 Boston Marathon in Boston, April 15, 2013. (AP Photo/Charles Krupa, File)

FILE - This photo released by the Federal Bureau of Investigation on April 19, 2013, shows Dzhokhar Tsarnaev, the man convicted of carrying out the April 2013 Boston Marathon bombing attack. (FBI via AP, File)