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Federal prosecutors are trying to piece together the complex web of influences that transformed a young man with no confirmed militant training or links, apparently acting with only the assistance of his younger brother, into a brutal bomber prepared to kill and maim in pursuit of a cause that remained largely unarticulated.

Tamerlan Tsarnaev has become the focal point of a global FBI investigation into whether any organised group or wider conspiracy lay behind last week's Boston Marathon bombings. The 26-year-old, who has been identified through fingerprinting as the man killed in the shootout with police in the Watertown suburb of Boston, is widely assumed to have been the mastermind of the marathon outrage, with his younger brother Dzhokhar Tsarnaev allegedly playing the role of junior partner.

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