BOSTON — It was a blow the immigrant boxer could not withstand: after capturing his second consecutive title as the Golden Gloves heavyweight champion of New England in 2010, Tamerlan Anzorovich Tsarnaev, 23, was barred from the national Tournament of Champions because he was not a United States citizen.

The cocksure fighter, a flamboyant dresser partial to white fur and snakeskin, had been looking forward to redeeming the loss he suffered the previous year in the first round, when the judges awarded his opponent the decision, drawing boos from spectators who considered Mr. Tsarnaev dominant.

From one year to the next, though, the tournament rules had changed, disqualifying legal permanent residents — not only Mr. Tsarnaev, who was Soviet-born of Chechen and Dagestani heritage, but several other New England contenders, too. His aspirations frustrated, he dropped out of boxing competition entirely, and his life veered in a completely different direction.

Mr. Tsarnaev portrayed his quitting as a reflection of the sport’s incompatibility with his growing devotion to Islam. But as dozens of interviews with friends, acquaintances and relatives from Cambridge, Mass., to Dagestan showed, that devotion, and the suspected radicalization that accompanied it, was a path he followed most avidly only after his more secular dreams were dashed in 2010 and he was left adrift.