While the younger brother prayed daily during lunch breaks at Rindge and Latin, and at least on occasion in his university dormitory, he never appeared especially devout, even telling one teacher, “I’m really not into that.” Up to his arrest, he drank and smoked marijuana — more marijuana than most high school or college students, friends said — despite what he said was Tamerlan’s clear disapproval.

The Dzhokhar that Mr. Mazaev and Mr. Umarov were allowed to see — in Mr. Umarov’s case, as recently as March — was the same Dzhokhar they had known for a decade.

Inside, however, some things were changing…

Just a year ago, Mr. Tsarnaev had wanted to become an engineer, and worried about his grades, said Sanjaya Lamichhane, a high-school wrestling teammate and former UMass Dartmouth classmate.

But as April began, Mr. Tsarnaev apparently declared that he no longer cared. After Mr. Tsarnaev emerged as a suspect in the bombing, Mr. Lamichhane said, a mutual friend from the University of Massachusetts recounted his last conversation with Mr. Tsarnaev, two weeks before the marathon. Mr. Tsarnaev told their friend, “God is all that matters. It doesn’t matter about school and engineering,” Mr. Lamichhane said. “He said, ‘When it comes to school and being an engineer, you can cheat easily. But when it comes to going to heaven, you can’t cheat.’ ”