The suspects involved in the Boston Marathon bombing were brothers from the Russian Caucasus who moved to Kazakhstan before coming to the United States several years ago, a source briefed on the investigation told CNN.
One of the brothers, identified by several sources as Tamerlan Tsarnaev, 26, studied engineering at Bunker Hill Community College but had taken off a year to train as a boxer, the source said Friday.
The source said a posting on a social media website under his name included the comment: "I don't have a single American friend. I don't understand them."
He died at a hospital overnight after a gun battle with police, authorities said. A source briefed on the investigation says Tsarnaev was wearing explosives and an explosive trigger when his body was recovered.
Several sources identified the other brother, who remained on the lam, as Dzhokhar Tsarnaev, 19.
The source briefed on the investigation added that it should not be assumed that the brothers were radicalized because of their origins in the Russian Caucasus.
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."
An official in the central Asian republic of Kyrgyzstan told CNN that the brothers were Kyrgyz passport holders, and used those passports when applying for green cards in the United States.
Many refugees from the Caucasus conflict received passports or refugee status in surrounding countries.
Two sources told CNN Dzhokhar Tsarnaev came to the U.S. as a tourist with his family in the early 2000s and later asked for asylum. He became a naturalized U.S. citizen in 2012. Tamerlan Tsarnaev was not a naturalized citizen. He came "a few years later" and was lawfully in the United States as a green-card holder.
A leader of the ethnic Chechen community in Kyrgyzstan told CNN that the Tsarnaev family left the republic more than a decade ago.
"There haven't been any Tsarnaev's living here in 10 or 15 years," said Adnan Djubrailov, in a phone call from Kyrgyzstan.
Dzhokhar Tsarnaev had worked as a lifeguard at a pool at Harvard University, said George McMasters, who hired him about two and a half years ago and said he was impressed with his work ethic. "He showed up on time, he watched the water, he rotated from position to position fine, got along well with others."
McMasters, who is the aquatic coordinator, said Tsarnaev gave no clue to a violent side. "He seemed like a very quiet, unassuming young man," he said. "It is very surprising and shocking to see the destruction that he has brought to the city."
Last year, McMasters was deployed to Afghanistan with the Army National Guard and, when he returned to the job in August, Tsarnaev was no longer on the staff or the schedule, he said.
Dzhokhar Tsarnaev attended Cambridge Rindge & Latin, a public high school, said friend Eric Mercado, who graduated a year behind the suspect.
"We hung out; we partied; we were good high school friends," Mercado told CNN.
"We're all, like, in shock. We don't really understand. There were no telltale signs of any kind of malicious behavior from Dzhokhar. It's all coming as a shock, really."
Mercado said he lived a block away from the suspect and had not known his older brother.
"To think that he's capable of something like this is beyond belief," Mercado said. The younger brother was registered at the University of Massachusetts-Dartmouth, which ordered its campus evacuated on Friday. The school is located 65 miles south of Cambridge, just west of New Bedford.
Larry Aaronson, a former teacher at Cambridge Rindge & Latin who lives near Dzhokhar Tsarnaev's residence in Cambridge, said he had taken pictures of the younger brother wrestling. "There is nothing in his character, in his deportment, in his demeanor that would suggest anything remotely capable of any of these things that he is now suspected of doing," Aaronson told CNN.
"He was so grateful to be here, he was so grateful to be at the school," he said. "He was compassionate, he was caring, he was jovial."
He described the suspect, whom he last ran into in the neighborhood a few weeks ago, as "a lovely, lovely kid."
The other brother, Tamerlan Tsamaev, was listed as a participant in the 201-pound class in a Salt Lake City 2009 Golden Gloves event.
Ruslan Tsarni, a Maryland man who said he was an uncle of the suspects, had no sympathy for them and no regret that his elder nephew was killed. "Good," he told CNN affiliate WBZ. "He got what he deserved."
Tsarni said Tamerlan Tsarnaev came to the United States as a child in 2000 or 2001, and that he last saw him in 2005 or 2006. "He's really been a quiet, nice boy," he said.
CNN's Clare Sebastian in London and Ivan Watson contributed to this report