Investigators continue to fill in the blanks, but one large question continues to hang over the terror attack during the Boston Marathon: Why? What drove Tamerlan Tsarnaev and his younger brother Dzhokhar to plant two crude homemade bombs near the finish line? The explosions killed three people and injured 282.

The evidence suggests Tamerlan, the older of the two, was motivated by extremist Islamic beliefs and a deep animus toward the United States. It is not clear what motive, if any, drove Dzhokhar. It is believed that they acted alone, but officials continue to trace all possible contacts to see if they are part of a larger terror network.

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