The schoolmates, teachers and neighbours of Dzhokhar and Tamerlan Tsarnaev said they saw little sign of radicalism - or anything extraordinary - to explain why the ethnic Chechen brothers would allegedly carry out the twin bombings that killed three people and wounded 176 at the Boston Marathon on Monday.
Tamerlan, 26, who dreamed of Olympic boxing glory and appeared to have become a more observant Muslim in recent years, was killed in a shootout with police late on Thursday. Dzhokhar, 19, who was a high school wrestler, was captured by police after a manhunt that virtually shut down Boston.
Twenty-four hours after the initial gunfight, the physical journey of the pair from Russia to Cambridge, Massachusetts, is fairly well-documented. But the psychological journey that might have led them to carry out the worst bomb attack on US soil since the plane hijackings on September 11, 2001, is largely an enigma.
Exactly what motivated the pair remains a mystery.
Four US government officials said they were unaware of any information in government databases that would have, before this week, flagged the Tsarnaev brothers as militants who might become involved in attacks.
But late on Friday, the FBI said it interviewed the elder brother in 2011, acting at the request of an unidentified foreign government.
The FBI's dealings with Tamerlan did not produce any "derogatory" information, and the matter was put "to bed," a US law enforcement source said, speaking on condition of anonymity.
The revelation was the first indication that the brothers were known to US security officials prior to Monday's bombings, US authorities said.
The Tsarnaev brothers, who have two sisters, are ethnic Chechen, from a predominately Muslim region of Russia where separatists fought two wars in the 1990s after the fall of the Soviet Union.
They were schooled in Dagestan, a neighbouring region that was drawn into Chechnya's violence during the 1990s and has since become the focal point for a simmering Islamist insurgency.
Their aunt, Maret Tsarnaeva, who lives in Toronto, said the family had refugee status in the United States.
The younger brother arrived in 2002 with his parents. At the time, his three siblings were in Kazakhstan but later united with Dzhokhar in the United States. The father, Anzor, later went back to Dagestan with the mother, who has traveled back and forth to the United States.
Their uncle, Ruslan Tsarni, who lives in Maryland, told reporters he had not spoken with Dzhokhar and Tamerlan since 2009.
"He put a shame on our family, he put a shame on the entire Chechen ethnicity," Tsarni said of Dzhokhar.
But their father, interviewed in Dagestan by CNN, insisted his sons were not responsible for the bombings.
"Someone framed them - being cowards they shot the boy dead," he said in Russian. "I honestly can't imagine who would do this. Whoever did it is a bastard."
Former classmates speculated that it might be the older brother who had influenced Dzhokhar. A YouTube account under Tamerlan's name featured two videos about terrorism.
Tamerlan appeared to become more religious in recent years, according to his aunt.
"He was not devout, practicing. But about three years ago he began praying five times a day," Tsarnaeva said in comments carried on CNN, adding that she had no problem with that change.
More than anything, Dzhokhar wanted to be popular, according to those who knew him in Cambridge, home of Harvard University and Massachusetts Institute of Technology, a city where students and academics mix easily with blue-collar workers and recent immigrants.
Friends and acquaintances say he laughed at everyone's jokes. He tried hard to get along with everybody. He used the word "dude." He liked hip hop. He was cheery, nervous and socially awkward - but not in a way that made people uncomfortable. And he didn't talk about politics much.
"Seriously, he was so, so normal, no accent, an all-American kid in every measurable sense of the word," said Nate Mann, 20, who was in the class above the younger Tsarnaev at Cambridge Rindge & Latin School.
The older brother, who was known as Timmy, appeared to be less social.
"I don't have a single American friend," Tamerlan was quoted as saying in a 2010 profile in "The Comment" magazine, published by Boston University's School of Communications. "I don't understand them."
Luis Vasquez, a 25-year-old youth counsellor who went to school with Tamerlan and lives two blocks away from the Tsarnaev house, remembers him differently.
"He was just a big friendly giant," Vasquez said. "He had a sense of humour."
Tamerlan had a wife, Katherine Russell, and a young daughter. His aunt said he was "very happy" about his child.
"We cannot begin to comprehend how this horrible tragedy occurred," the Russell family said in a statement. "In the aftermath of the Patriot's Day horror, we know that we never really knew Tamerlan Tsarnaev. Our hearts are sickened by the knowledge of the horror he has inflicted."
Tamerlan had been a part-time accounting student at Bunker Hill Community College in Boston. He was enrolled there for three semesters - fall 2006, spring 2007 and fall 2008.
"He wasn't even close" to getting a degree, said Patricia Brady, a spokeswoman for the college.
After winning a Golden Gloves amateur boxing competition in nearby Lowell, Massachusetts, in 2004, Tamerlan told the local newspaper, "I like the USA ... 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."
Tamerlan was quoted in "The Comment" magazine as saying he wanted to participate in the Olympics and would "rather compete for the U.S. than Russia" if he could not represent an independent Chechnya.
He neither smoked nor drank, and said "God said no alcohol," according to the magazine's profile of the amateur boxer.
Those who knew the younger brother painted a picture of a youth who bore none of the telltales of an immigrant: Dzhokhar had virtually no accent. He dressed in sweats and a sweatshirt. He did not hang with anybody in the school's Muslim circle.
The brothers' public high school, Cambridge Rindge and Latin, is one of the most diverse in the country, its student body ranging from the children of immigrants to those of Harvard professors.
It counts actors Matt Damon and Ben Affleck, poet E.E. Cummings and basketball star Patrick Ewing among its alumni.
"I always sympathized with him because he was, I'd say, a kid that just really wanted to be accepted," said Taylor Conlin, who played on the Rindge lacrosse team with Dzhokhar. "He did it in a very humble way, but he just tried to hang out with the cool kids."
Former classmates were astonished to hear that "sweet, considerate" Dzhokhar, who later enrolled at the University of Massachusetts at Dartmouth, could have been involved in the explosions.
