Apr 19, 2013 | 1715 GMT
The two Chechen suspects are part of a broader post-9/11 trend of decentralized militant activity.
STAN HONDA/AFP/Getty Images
Photographs of Chechen brothers Dzhokhar Tsarnaev, 19, and Tamerlan Tsarnaev, 26, two suspects wanted in the Boston Marathon bombings released by the FBI on April 18, 2013, in Boston, Mass.