LINE OF DUTY

Killed MIT Cop Had 'A Heart of Gold'

26-year-old Sean Collier was shot by the Boston bombing suspects.

Now that the two Boston marathon bombing suspects have been captured, one dead, the other alive and in serious condition, speculation swirls around what motivated Tamerlan and Dzhokhar Tsarnaev, brothers and Chechnyan immigrants, to attack a massive crowd of innocent people. What information is available about the two brothers suggests that Tamerlan, the older brother, may have influenced Dzhokhar to participate in the bombing. Tamerlan, a 26-year-old boxer who'd once aspired to box for the U.S. Olympic team, seemed to struggle with life in America more than his younger brother. Tamerlan was still trying to become a citizen at the time of the bombings, held back by a domestic violence complaint from a girlfriend. Dzhokhar, 19, whose parents and friends find his involvement in the bombings impossible to believe, looked up to his older brother.