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

Mr. Tsarnaev was devastated. He was not getting any younger. And at that point, he was more than a year away from being even eligible to apply for American citizenship, and there appeared to be a potential obstacle in his path.

The previous summer, Mr. Tsarnaev had been arrested after a report of domestic violence.