BOSTON (CNN) -- By most appearances, the Tsarnaev brothers were well on their way to achieving the American dream in their adoptive country before the FBI identified them as suspects in Monday's bombings near the finish line of the Boston Marathon.
Tamerlan Tsarnaev, 26, studied engineering at Bunker Hill Community College just outside Boston but had taken the year off to train as a boxer, sources told CNN. In 2009, he had competed in a Golden Gloves match -- heavyweight division -- in Salt Lake City, losing in the first round.
He died at a Boston hospital overnight after a gun battle with police. A source briefed on the investigation said he was wearing explosives and an explosive trigger when his body was recovered.
Dzhokar Tsarnaev, 19, is enrolled at the University of Massachusetts - Dartmouth and was the object of a massive manhunt on Friday. In 2011, he was one of 45 high school seniors awarded a $2,500 scholarship by the City of Cambridge.
His friends expressed shock about his suspected involvement in the bombings, which killed three people and wounded scores more.
"We hung out; we partied; we were good high school friends," Eric Mercado told CNN about Dzhokar, who graduated a year ahead of him from Cambridge Rindge & Latin, an ethnically diverse public high school.
"There were no telltale signs of any kind of malicious behavior from Dzhokar. It's all coming as a shock, really."
But there were hints of unhappiness. A posting on a social media website under the elder brother's name included the comment: "I don't have a single American friend. I don't understand them."
And in 2009, Tamerlan Tsarnaev, then 22, was arrested for domestic assault and battery after allegedly assaulting his girlfriend, according to Cambridge Police records cited by the website spotcrime.com.
The family was from the volatile Russian Caucasus and had lived for several years in the United States.
Though the motive for the terrorist attack at the race is still unknown, the source briefed on the investigation added that it should not be assumed that the brothers were radicalized because of their origins.
The spokesman for Chechen President Ramzan Kadyrov said the brothers had not been connected with the Chechen Republic for many years, Russia's semi-official Interfax news agency reported Friday.
"According to preliminary information, coming from the relevant agencies, the Tsarnaev family moved many years ago out of Chechnya to another Russian region," press secretary Alvi Kamirov told Interfax. "After that they lived for some time in Kazakhstan, and from there went to the U.S. where the family members received a residence permit. Therefore the individuals concerned did not live as adults in Chechnya."
Kadyrov said any blame lay at the feet of the United States. "It's all America's fault because these kids were brought up in America, not Chechnya," he said, according to Kommersant newspaper.
An official in the central Asian republic of Kyrgyzstan told CNN that the brothers held Kyrgyz passports, which they had used to apply for green cards in the United States.
Many refugees from the Caucasus conflict have received passports or refugee status in surrounding countries.
A leader of the ethnic Chechen community in Kyrgyzstan told CNN that the Tsarnaev family left the republic long ago.
"There haven't been any Tsarnaevs living here in 10 or 15 years," Adnan Djubrailov said in a phone call from Kyrgyzstan.
In a statement, the State Committee for National Security of Kyrgyzstan sought to distance itself from the family.
"The Tsarnaev family lived in the town of Tokmok and 12 years ago it moved to live in the Republic of Daghestan in the Russian Federation, from where it emigrated to the USA," it said.
"Given that the suspects left the republic at the ages of 8 and 15, the State Committee for National Security of Kyrgyzstan considers it inappropriate to link them to Kyrgyzstan.
The headmaster of the school in Daghestan where Dzhokar Tsarnaev attended first grade told Reuters that the family -- which included two brothers and two sisters -- arrived in 2001 from Kyrgyzstan as refugees.
"The whole family arrived together and left together," said Emirmagomed Davydov, headmaster at School #1. "They attended the fifth and the eighth grades. The youngest (Dzhokar Tsarnaev), was admitted to the first grade, and without even finishing the first grade he along with his family left in spring, in March."
A federal official said the younger brother arrived on a tourist visa in the United States on July 1, 2002, at age 8. After seeking asylum, he was granted citizenship on September 11, 2012.
Their father has since returned to Russia.
His brother, Ruslan Tsarni, told reporters outside his home in Montgomery County, Maryland, that he had not seen the Tsarnaev family since December 2005 and last spoke with them in 2009.
Asked what might have motivated his nephews to carry out such an attack, he said: "Being losers; hatred to those who