BOSTON (AP) — Tamerlan Tsarnaev was an amateur boxer with muscular arms and enough brio to arrive at a sparring session without protective gear. His younger brother Dzhokhar was popular in high school, won a city scholarship for college and liked to hang out with Russian friends off-campus.

Details of two lives, suddenly infamous, came to light Friday. Overnight, two men previously seen only in grainy camera images were revealed to be ethnic Chechen brothers suspected in a horrific act of terrorism. Tamerlan was dead; his 19-year-old brother would be captured after a furious manhunt that shut down much of Boston.

But the details of their lives shed precious little light on the most vexing question: Why would two brothers who came to America a decade ago turn on their adopted home with an attack on a cherished tradition, the Boston Marathon?

The Tsarnaev family arrived in the United States, seeking refuge from strife in their homeland. ‘‘Why people go to America? You know why,’’ the father, Anzor Tsarnaev, said in an interview from Russia, where he lives now. ‘‘Our political system in Russia . Chechens were persecuted in Kyrgyzstan, they were problems.’’ The family had moved from Kyrgyzstan to Dagestan, a predominantly Muslim republic in Russia’s North Caucasus that has become an epicenter of the Islamic insurgency that spilled over from Chechnya.

The father set up as an auto mechanic, and the two boys (there were two sisters, too) went to school. Dzhokhar, at least, attended the Cambridge Rindge and Latin school, a prestigious public school just blocks from Harvard Yard.

From there, the boys’ paths diverged somewhat — at least for a while.

Tamerlan, who was 26 when he was killed overnight in a shootout, dropped out after studying accounting at Bunker Hill Community College for just three semesters.

‘‘I don’t have a single American friend. I don’t understand them,’’ he was quoted as saying in a photo package that appeared in a Boston University student magazine in 2010.

He identified himself then as a Muslim and said he did not drink or smoke: ‘‘God said no alcohol.’’ He said he hoped to fight for the U.S. Olympic team and become a naturalized American.

As a boxer, he was known for his nerve. ‘‘He’s a real cocky guy,’’ said one trainer who worked with him, Kendrick Ball. He said the young man came to his first sparring session with no protective gear. ‘‘That’s unheard of with boxing,’’ Ball said. But he added: ‘‘In this sport, you've got to be sure of yourself, you know what I mean?’’

More recently, Tamerlan — married, with a young daughter — became a more devout Muslim, according to his aunt, Maret Tsarnaeva. She told reporters outside her Toronto home Friday that the older brother had taken to praying five times a day.

In 2011, the FBI interviewed Tamerlan at the behest of a foreign government, a federal law enforcement official said, speaking anonymously. The officials would not say what country made the request or why, but said that nothing derogatory was found.

Albrecht Ammon, 18, lived directly below the apartment of the two suspects. He said he recently saw Tamerlan in a pizzeria, where they argued about religion and U.S. foreign policy. He quoted Tsarnaev as saying that many U.S. wars are based on the Bible, which is used as ‘‘an excuse for invading other countries.’’

During the argument, Ammon said, Tsarnaev told him he had nothing against the American people, but he had something against the American government. ‘‘The Bible was a cheap copy of the Koran,’’ Ammon quoted Tsarnaev as saying.

Tamerlan traveled to Russia last year and returned to the U.S. six months later, government officials told The Associated Press. More wasn’t known about his travels.

According to law enforcement records he was arrested, in 2009, for assault and battery on a girlfriend; the charges were dismissed. His father told The New York Times that the case thwarted Tamerlan’s hopes for U.S. citizenship.

Meanwhile, the mother of the suspects, Zubeidat Tsarnaev, was heard from only in an audio interview broadcast on CNN, defending her sons and calling the accusations against them a setup. She said she had never heard a word from her older son about any thinking that would have led to such an attack. ‘‘He never told me he would be on the side of jihad,’’ she said.