Dzhokhar, a baby-faced 19-year-old who was captured late Friday, was an all-star high school wrestler and a student at the University of Massachusetts whose Twitter feed @J_tsar resembles that of an average American teenager.
Neighbors and friends interviewed by US television networks described him as hard-working and kind, expressing shock at his alleged involvement in Monday's bombings that killed three people and wounded over 180.
"He was a lovely, lovely kid," Larry Aaronson, a neighbor, told CNN when asked about Dzhokhar.
"You get people that say, 'Oh, I know him. He's not capable of this.' That's what I'm saying to you. I am just one of those people."
While Dzhokhar was active on Twitter, firing off hundreds of messages about his life and thoughts, less is known about Tamerlan, his elder brother who was killed in a shootout with police.
A 26-year-old engineering student who loved boxing, Tamerlan also apparently attracted the attention of the US authorities on a previous occasion.
The FBI said its agents had interviewed him in 2011 at the request of a foreign government, an investigation that found "no derogatory information," according to an official from the Federal Bureau of Investigation.
The FBI official declined to identify the country involved or the nature of the probe.
A page on the VKontakte social media website, Russia's equivalent to Facebook, says Dzhokhar went to elementary school in the Dagestani city of Makhachkala in southern Russia from 1999-2001.
Dagestan borders the North Caucasus region of Chechnya, which has been ravaged by two back-to-back wars since 1994 between Russia's army and increasingly Islamist-leaning separatist rebels.
The Tsarnaev family appears to have left the volatile Caucasus region, which still sees occasional fighting, during the wars and subsequently spent time in Central Asia.
"We're Muslims, we're Chechens, we're ethnic Chechens," Ruslan Tsarni, an uncle of the suspects, told US media.
"Somebody radicalized them," he said of his nephews. "It's not my brother."
Tsarni said his nephews arrived in the United States from Kyrgyzstan in 2003 and were given asylum, but he called them "losers" who could not integrate into American life.
Asked why they might have turned to terrorism, he said: "Hatred to those who were able to settle themselves. These are the only reasons I can imagine."
An online photo essay by Boston University student Johannes Hirn featuring Tamerlan entitled "Will Box for Passport" seems to suggest that he did have some trouble adapting to the United States.
"I don't have a single American friend, I don't understand them," Tamerlan is quoted as saying under one of several pictures of him boxing at the Wai Kru Mixed Martial Arts Center.
The website says Tamerlan, who was studying engineering at Bunker Hill Community College, had taken a year off to train for the National Golden Gloves competition in Salt Lake City, Utah.
On the website, Tamerlan is quoted as saying he aspires to be an Olympic boxer, but would rather compete for the United States than for Russia in the absence of an independent Chechnya.
He is quoted as describing himself as "very religious" and saying, "God said no alcohol."
"There are no values anymore," he is quoted as saying. "People can't control themselves."
But another caption says his favorite movie is "Borat," and one picture shows him with a blonde woman whom he describes as his half-Italian, half-Portuguese girlfriend, saying she converted to Islam.
"She's beautiful, man!" he says.
A YouTube account in his name created on August 17, 2012 linked to videos of a radical Australian preacher, Feiz Mohammad, and had a playlist entitled "terrorists," according to the Washington-based SITE Intelligence Group.
SITE said the videos in the playlist had been removed, but a saved page indicates they were related to Dagestan militants.
Dzhokhar's VKontakte page says he graduated in 2011 from Cambridge Rindge and Latin School, a public high school, identified "Islam" as his world view, and "career and money" as his main goals in life.
It also lists information about Chechnya and Islam, and relates jokes about the unfair treatment of Muslims in the Caucasus region.
"They have this riddle in school. There is a car. Inside are a man from Dagestan, one from Chechnya and another from Ingushetia... Who is driving the car? The police," it says.
The boys' father, Anzor Tsarnaev, speaking to Russia's Interfax news agency, expressed doubt over his sons' alleged involvement in the marathon bombings.
He said his children "were set up by the secret services because they are practicing Muslims."