A CLASSIC DYAD?:

The two young brothers from Cambridge seemed to be on promising paths, one a scholarship student at college, the other fighting for a national title in amateur boxing.

And then, apparently with little warning, they veered violently off track, deep into the darkness, setting off deadly bombs, authorities are convinced, at one of Boston's most iconic and joyful events.

To those who knew them, the apparent transformation of Tamerlan Tsarnaev, 26, and Dzhokhar Tsarnaev, 19 -- ethnic Chechens, born in the former Soviet territory now known as Kyrgyzstan and transplanted to a working-class Inman Square neighborhood -- seemed almost inconceivable.

But as friends and neighbors pieced together recollections of the terrorism suspects and their family, a picture emerged of an older brother who seemed to grow increasingly religious and radical -- and who may have drawn his more easygoing younger brother into a secret plot of violence and hatred.

"I used to warn Dzhokhar that Tamerlan was up to no good," Zaur Tsarnaev, who identified himself as a 26-year-old cousin, said in a phone interview from Makhachkala, Russia, where the brothers briefly lived. "[Tamerlan] was always getting in trouble. He was never happy, never cheering, never smiling. He used to strike his girlfriend. . . . He was not a nice man."

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