AP Tamerlan Tsarnaev, in a 2010 photo from The Sun of Lowell provided by AP, and Dzhokhar Tsarnaev, in an FBI photo.

One became an American citizen last year on Sept. 11. The other was a boxer who once said: “I like the USA.”

The two suspects in the attack on the Boston Marathon — one killed, one captured alive after a daylong manhunt — are brothers of Chechen origin had come to the United States a decade ago, law enforcement officials told NBC News.

A complicated portrait of the two Tsarnaev brothers is coming into view. Again and again, people who knew them use words like "normal" and even "outgoing" -- and say they never hinted at extremism. NBC's Ann Curry reports on two young men who seemed to disappear in the crowd--until this week.

The suspect at large for most of Friday was Dzhokhar Tsarnaev, 19, who was born in Kyrgyzstan and became a naturalized American citizen on Sept. 11, 2012, according to documents obtained by NBC News.

Authorities hunted him door-to-door in the Boston suburbs, ordered a lockdown that paralyzed a region of more than 1 million people for most of the day, and took him after darkness fell and he holed up inside a boat outside a home.

His older brother, Tamerlan, 26, was killed overnight after an extraordinary crime spree: The brothers shot and killed a college security officer, carjacked an SUV and hurled explosives at police in Watertown, Mass., authorities said.

As the anxious manhunt dragged on, profiles of the brothers began to emerge. People who know the younger Tsarnaev described him as bright, studious and quiet, with ambitions in medicine. The older was a boxer who said he dreamed of joining the U.S. Olympic team but had no American friends, and who was married with a young daughter.

An official also suggested that a foreign government had expressed terrorism concerns about the elder Tsarnaev to the FBI two years ago, and a cousin told The Boston Globe that the older brother was a bad influence on the younger.

Both men were active on social media. Dzhokhar cracked jokes and made pop-culture references. Tamerlan had a YouTube page that featured videos about Islamic radicalism.

Authorities were not sure of a motive in the marathon attack, which killed three people and injured 176 on Monday. 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, a separatist region that has warred with Russia for independence and launched devastating terror strikes, is predominantly Muslim.

“Somebody radicalized them, but it wasn’t my brother,” the men’s uncle, Ruslan Tsarni, told reporters Friday from Montgomery Village, Md. He said the two brothers 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.”

In Russia, the brothers’ father, Anzor Tsarnaev, told the newspaper Izvestia that his children had been set up.

Ruslan Tsarni speaks out about his relationship with his nephews, who he says he hasn't seen in years, saying "somebody radicalized them" and "I just wanted my family to be away from them."

Dzhokhar Tsarnaev had a Massachusetts driver’s license, was enrolled at the University of Massachusetts Dartmouth and was living in the Boston suburb of Cambridge. He was the suspect in the white hat in surveillance photos from the marathon released Thursday by the FBI, authorities said.

Speaking from Russia, the father said that Dzhokhar was a diligent student who dreamed of becoming a great doctor.

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.”

Slideshow: Search for suspects in Boston Marathon bombings / A tense night of police activity that left a university officer dead on campus just days after the Boston Marathon bombings and amid a hunt for two suspects caused officers to converge on a neighborhood outside Boston, where residents heard gunfire and explosions. Launch slideshow

“I can’t tell you enough what a beautiful young man this was,” she said.

He was also active on Twitter. In a post two days after the bombing, he wrote, “Ain’t no love in the heart of the city, stay safe people,” a reference to a Jay-Z lyric. One day later, he wrote, “I’m a stress free kind of guy.”

His account has been dormant since Wednesday, when he retweeted a post from a Muslim scholar: “Attitude can take away your beauty no matter how good looking you are or it could enhance your beauty, making you adorable.”

The city of Cambridge awarded Dzhokhar Tsarnaev a $2,500 scholarship toward college in 2011. The scholarships were for students at Cambridge Rindge and Latin School, part of the public school system in Cambridge, 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.

Slideshow: Search for suspects in Boston Marathon bombings Dominic Chavez / EPA A tense night of police activity that left a university officer dead on campus just days after the Boston Marathon bombings and amid a hunt for two suspects caused officers to converge on a neighborhood outside Boston, where residents heard gunfire and explosions. Launch slideshow

The other brother, Tamerlan, who was killed in the firefight with law enforcement, was 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.

In 2011, a foreign government expressed concern to the FBI that Tamerlan Tsarnaev could have ties to terrorism, but the FBI, after taking investigative steps, found no such links and reported the findings back to the foreign government, according to an official familiar with the matter.

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 had 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 said then that his first love was music, and that he played the piano and violin. That was also when he told the newspaper: “I like the USA.”

“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 boxing coach, John Curran, described him to NBC News as quiet and courteous and said that he was “flabbergasted” by the news.

His family appeared to be as well: In a statement handed to reporters through the door of a home in Rhode Island, the family of Tamerlan Tsarnaev’s wife, Katherine Russell, said it was clear that they never really knew him.

“Our hearts are sickened by the knowledge of the horror he has inflicted,” the statement said.

Neighbors said that Russell graduated high school in 2007, went to college in Boston and has been living in North Kingston, R.I., with her daughter, who is about 3. Paula Gillette, a neighbor, told NBC News that Russell began wearing Islamic dress a few years ago and rarely left the house, and that her husband often came home for weekends in a car with Massachusetts plates.

Both men were believed to have entered the country with their family in 2002 or 2003, when the Tsarnaev 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.

The men have a sister who lives in West New York, N.J. Police swarmed her home Friday morning and were seeing removing a computer, NBC New York reported. Earlier in the day, she told reporters: “I’m not OK, just like everybody else is not OK. No one is OK. It’s very shocking.”

Reuters reported that the Tsarnaev brothers were schooled in Dagestan, a region drawn into Chechen violence during the 1990s. Their mother has traveled back and forth to the United States over the past decade. The father told Izvestia that he has a tumor and returned to Russia to die.

Chechnya 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.

Pete Williams, Michael Isikoff, Richard Engel, Bill Dedman, Tracy Connor, Konrad Jankowski and Eun Kyung Kim of NBC News contributed to this report. The Associated Press also contributed.

Related:

Who are the brothers accused of the Boston Marathon bombing?

Chechen insurgents deny any link to marathon bombing

What we know: Timeline of terror hunt

‘Dedicated officer’ gunned down at MIT

Boston bombing spurs Senate debate on tighter immigration screening

This story was originally published on