One was a pot-loving student, the other had begun exploring Islam in an effort to turn his life around. So how did Dzhokhar and Tamerlan Tsarnaev end up at the Boston Marathon finishing line with a willingness to kill?
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The moment of the explosion at the Boston Marathon Photo
5:39PM BST 27 Apr 2013
As they hunched over the twisted bomb parts and pieces of shrapnel, some still bloody from where they cut through flesh, the FBI technicians were finding a grimly familiar pattern.
Elbow pipes from common plumbing systems. Specially-picked rounded bearings and nails of a short but disfiguring length. Gunpowder, cannibalised from the kind of large but everyday fireworks readily on sale in most American states.
The precise ingredients used by the Boston bombers had been selected with rigid fealty to the instructions laid out in Inspire, al-Qaeda's online magazine which offers would-be jihadists around the world a simple path to improvising explosives.
Department of Homeland Security 2010 warning on Pressure Cookers
The close attention the Tsarnaev brothers appear to have paid to Inspire's detailed instructions has given new urgency to America's hunt for its editor, an unknown Islamist who goes by the name "Yahya Ibrahim".
Like his predecessor, Samir Khan, who died in a drone strike in Yemen in 2011, Ibrahim appears to be a fluent English speaker; and the close match between the magazine's instructions and the bombs built by the Boston bombers appears to confirm the claim to his interrogators by Dzhokhar Tsarnaev, the surviving brother, that Inspire was critical in teaching the American-raised Chechens how to carry out their attack.
But while the FBI forensic experts have the relatively straightforward task of figuring out how Tamerlan and Dzhokhar Tsarnaev managed to kill three and wounded more than 200, their investigative colleagues in Boston and 4,000