BOSTON — 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.

Ammon quoted Tamerlan as saying that many U.S. wars are based on the Bible, which is used as “an excuse for invading other countries.” Tamerlan told him he had nothing against the American people, but he had something against the American government.

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.

Her younger son was described by friends as well-adjusted and well-liked in both high school and college, though at some point in college, his academic work reportedly suffered greatly.

“I’m in complete shock,” said Rose Schutzberg, 19, who graduated high school with Dzhokhar and now attends Barnard College in New York. “He was a very studious person. He was really popular. He wrestled. People loved him.”

In fact, Schutzberg said, she had “a little crush” on him in high school. “He’s a great guy,” she said. “He’s smart, funny. He’s definitely a really sweet person, very kind hearted, kind soul.”

Dzhokhar was on the school’s wrestling team. And in May 2011, his senior year, he was awarded a $2,500 scholarship from the city to pursue higher education, according to a news release at the time. That scholarship was celebrated with a reception at city hall.

The New Bedford Standard-Times reported that Dr. Brian Glyn Williams, who teaches Chechen history at the University of Massachusetts at Dartmouth, said he had tutored Dzhokhar in the subject when he was in high school.

“He was learning his Chechen identity, identifying with the diaspora and identifying with his homeland,” Williams said, adding that Dzhokhar “wanted to learn more about Chechnya, who the fighters were, who the commanders were.”

Dzhokhar went on to attend UMass-Dartmouth, according to university officials. He lived on the third floor of the Pine Dale dormitory. Harry Danso, who lives on the same floor, told the AP he saw him in a dorm hallway this week.

“He was regular, he was calm,” said Danso.

The school would not say what he was studying. The father of the suspects, Anzor Tsarnaev, told the AP his younger son was “a second-year medical student,” though he graduated high school in 2011.

“My son is a true angel …,” he said by telephone from the Russian city of Makhachkala. “He is such an intelligent boy. We expected him to come on holidays here.”

Still, The New York Times reported that a college transcript revealed that he was failing many of his college classes. In two semesters in 2012 and 2013, he got seven failing grades, including F’s in Principles of Modern Chemistry, Intro American Politics, and Chemistry and the Environment.

Dzhokhar’s page on the Russian social networking site Vkontakte says that before moving to the United States, he attended School No. 1 in Makhachkala, the capital of Dagestan, and he describes himself as speaking Chechen as well as English and Russian. His world view is described as “Islam” and he says his personal goal is “career and money.”

Deana Beaulieu, 20, lives two blocks away from the suspects’ home on Norfolk Street, went to high school with Dzhokhar and was friendly with his sister.

Beaulieu says she doesn’t recall Dzhokhar expressing any political views. “I thought he was going to branch off to college, and now this is what he’s done. … I don’t understand what the hell happened, what set him off like this.”

Florida Addy, 19, of Lynn, Mass., said she lived in the same college dorm with Dzhokhar this year and was on the same floor last year. She called him “drug” (pronounced droog), the Russian word for friend, a word he taught her.

Addy said she saw Dzhokhar last week, when she bummed a cigarette from him. They would occasionally hang out in his room or at the New Bedford apartment of Russian students he knew. He generally wore a hoodie or a white t-shirt and sweatpants, and spent a lot of his time with other kids from Russia.

She described him as down to earth and friendly, even a little mysterious, but in a charming way. She had just learned that he had a girlfriend, although she did not attend the university.

“He was nice. He was cool. I’m just in shock,” she said.

Tim Kelleher, a wrestling coach for a Boston school that competed in 2010 against Dzhokhar’s team, said the young man was a good wrestler, and that he’d never heard him express any political opinions.

“He was a tough, solid kid, just quiet,” said Kelleher, now a Boston public school teacher.

Dzhokhar’s uncle, too, was surprised by his suspected involvement in the attack — much more, he said, than by his brother’s. “It’s not a surprise about him,” Ruslan Tsarni, who lives in Maryland, said of Tamerlan. “The younger one, that’s something else.” He said the family had placed all its hopes with Dzhokhar, hoping he would be a doctor.

Tamerlan was more defined by sports, namely boxing. USA Boxing spokeswoman Julie Goldsticker said Tamerlan registered with the group as an amateur boxer from 2003 to 2004, and again from 2008 to 2010. He competed as a heavyweight in the National Golden Gloves competition in Salt Lake City on May 4, 2009, losing his only bout.

In photographs that appeared in the student magazine, including one in which he posed with his shirt off, Tamerlan has the muscular arms of a boxer, and is dressed in flashy street-clothes that he said were “European style.”

In another window onto his personality, his Amazon wish list — traced by the AP using an email address on his public record report — includes books on organized crime, document forgery, the conflict in Chechnya, and two self-help books, including Dale Carnegie’s “How to Win Friends & Influence People.”

Gene McCarthy, who trained Tamerlan at the Somerville Boxing Club, described him as a “nice kid” who already was a good fighter before he showed up at the gym years ago.

“He never lost a bout for me,” McCarthy said. “He had some skills from his father before he showed up in my gym.” McCarthy described the young man as “very intelligent” and recalled that he also played classical piano.

In Kyrgyzstan, the former Soviet republic where the family lived before it moved to Dagestan, Leila Alieva, a former schoolmate, remembers an educated family and a nice boy.

“He was … a good student, a jock, a boxer. He used to win all the (boxing) competitions in town,” she said. “I can’t believe they were involved in the explosions, because Tamerlan was a very positive guy, and they were not very Islamist. They were Muslim, but had a secular lifestyle.”

In a local news article in 2004, Tamerlan spoke about his boxing and his views of America.

“I like the USA,” Tamerlan was quoted as saying in The Sun of Lowell, Mass. “America has a lot of jobs. That’s something Russia doesn’t have. You have a chance to make money here if you are willing to work.”