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It's been four years since brothers Tamerlan and Dzhokhar Tsarnaev bombed the Boston Marathon. Since then there have been a number of other terror attacks — in Paris, Manchester, Brussels and Paris again — where the attackers either used bombs similar to the ones used in Boston, or where the attackers were brothers.
So what have we learned from the marathon bombing, and what are the controversial questions? One of them: Who is the other person seen on Tamerlan Tsarnaev's immigration form (pictured above)?
This segment aired on June 16, 2017.