A man referred to as "Suspect #2", taken at a convenience store in Cambridge, MA late Thursday night. (FBI/NBC News)
WATERTOWN, MA (NBC) -- One became an American citizen last year on Sept. 11. The other was a boxer who once said: "I like the USA."
The two known suspects in the attack on the Boston Marathon - one killed, one on the loose - are brothers with a background in the separatist Russian republic of Chechnya, law enforcement officials told NBC News.
WATCH LIVE CONTINUING COVERAGE HERE
MORE | Interactive live coverage here
The suspect at large early Friday was Dzhokhar Tsarnaev, 19, born in Kyrgyzstan, who became a naturalized American citizen on Sept. 11, 2012, according to documents obtained by NBC News. He had a Massachusetts driver's license and was living in the Boston suburb of Cambridge. He was the suspect in the white hat in surveillance photos from the marathon released by the FBI, authorities said.
Authorities were hunting him door-to-door in the Boston suburbs, and more than 1 million people were ordered to stay indoors in a lockdown that paralyzed the region.
GALLERY | Friday's hunt for bombing suspects in Mass.
MORE | Full Boston Marathon bombing coverage
RELATED | Bombing suspects of Chechen origin
His brother, Tamerlan Tsarnaev, was killed overnight after an extraordinary crime spree: The brothers shot and killed a college security officer, carjacked an SUV and hurled explosives as police in Watertown, Mass., authorities said.
Authorities were not sure of a motive and cautioned that other people may be involved. NBC News learned that counterterrorism officials were examining possible links between the brothers and the Islamic Jihad Union of central Asia, a terrorist group. Chechnya is predominantly Muslim.
RELATED | Police release pictures of bombing suspects
PHOTOS | FBI photos of bombing suspects
PHOTOS | Boston interfaith prayer service
VIDEO | FBI video of bombing suspects
"Somebody radicalized them, but it wasn't my brother," the men's uncle, Ruslan Tsarni, told reporters Friday from Montgomery Village, Md. He encouraged his nephew to turn himself in and said the two had brought shame on Chechens. He said that he had encouraged his own family to stay away from that part of the family.
"What I think was behind it: Being losers," he said. "Of course we're ashamed."
The brothers' father, speaking from Russia, told The Associated Press that Dzhokhar was "a true angel" with an interest in medicine. He was registered as a student at the University of Massachusetts Dartmouth, the school said. He was awarded a $2,500 city scholarship toward college two years ago.
Sierra Schwartz, who identified herself as a high school friend of Dzhokhar Tsarnaev, told NBC News that he had lots of friends and did not seem to brood. A lifeguard described him as hilarious.
"He was a nice guy. He was shy," Schwartz said. "It was almost physically painful to even call him nice now after this absolute tragedy that happened, but at the time, as we knew him, he was funny."
Robin Young, who said her nephew was on the wrestling team with Dzhokhar Tsarnaev, told NBC News that he was "just a light, airy, curly-haired kid."
"I can't tell you enough what a beautiful young man this was," she said.
The city of Cambridge awarded Dzhokhar Tsarnaev the scholarship in 2011, according to The Boston Globe. The scholarships were for students at Cambridge Rindge and Latin School, part of the Cambridge public school system. Cambridge is a melting-pot city of about 100,000, fairly well-off and home to Harvard University and the Massachusetts Institute of Technology.
On a Russian social media site, Dzhokhar Tsarnaev identified his religion as Islam and his priorities as career and money. He posted links to stories about the brutality of the ruling regime in Syria and to a YouTube video of himself doing impressions, to the amusement of his brother, of dialects from regions around Chechnya.
The other brother, Tamerlan, killed in the firefight with law enforcement, was identified as 26 and born in Russia. He became a legal permanent resident in 2007, the officials said. He was the suspect in the black hat in the FBI photos.
Tamerlan Tsarnaev studied at Bunker Hill Community College in Boston and wanted to become an engineer, according to a profile that appeared in a Boston University magazine in 2010. He said that he hoped to become an American citizen and one day join the U.S. Olympic boxing team.
He told the magazine that his family fled Chechnya in the 1990s because of the conflict there, and that he lived in Kazakhstan. While he had been in the United States for several years by that point, he said in the profile: "I don't have a single American friend. I don't understand them."
He also said that he was a Muslim who did not smoke or drink.
Travel records obtained by NBC New York showed that Tamerlan Tsarnaev left the country for six months, from Jan. 12 to July 17, 2012, for Russia. The records show that it was not until 6 a.m. Friday that he was labeled by American officials to be "a person or instrument that may pose a threat to the security of the United States."
Tamerlan Tsarnaev boxed in a 2004 tournament as part of a program called Golden Gloves, according to The Lowell Sun newspaper. He told the newspaper then: "I like the USA."
He said that his first love was music, and that he played the piano and violin.
"America has a lot of jobs," he said. "That's something Russia doesn't have. You have a chance to make money here if you are willing to work."
His former wrestling coach, John Curran, described him to NBC News as quiet and courteous and said that he was "flabbergasted" by the news.
The father, Anzor Tsarnaev, said that he had seen on television that his son was killed.
"They were set up!" he exclaimed, according to the AP.
Both men were believed to have entered the country with their family in 2002 or 2003, when the family sought asylum. Law enforcement officials initially told NBC News that they may have had military experience, but the nature was not clear. Later in the morning, U.S. Army officials told NBC News that no one matching either name had served in the active-duty Army, or the reserves.
Chechnya is a predominantly Muslim part of Russia with a turbulent history. It declared independence in 1991, after the breakup of the Soviet Union. Chechnya fought wars with Russia for much of the 1990s, and Chechens have been involved in terrorist attacks in Russia in the years since.
In 2002, Chechen militants seized a Moscow theater and held 800 people hostage for two days. Special forces raided the building and killed 41 hostage-takers; 129 hostages were killed, mostly from gas used by Russian forces.
In 2004, Chechen insurgents took hundreds of hostages in the Russian town of Beslan. The siege came to a bloody end two days later, and 330 people, about half children, were killed.