Similar to many atrocities, those who knew the perpetrators expressed disbelief. But in the case of Dzhokhar, friends and classmates seem to be genuinely shocked. He was quiet but not antisocial. He partied and used to smoke a lot of marijuana. He was good at sports, good at school and good at talking with people.
His older brother, it seems, was less successful - and more aggressive.
"I used to warn Dzhokhar that Tamerlan was up to no good," their cousin Zaur Tsarnaev, 26, told the Boston Globe. "He [Tamerlan] was never happy, never cheering, never smiling. He used to strike his girlfriend. He hurt her a few times. He was not a nice man. I don't like to speak about him. He caused problems for my family."
A relative said Tamerlan had recently become more religious, taking up the Islamic practice of praying five times a day.
He flew to Russia last year and didn't return to the U.S. for six months, officials said. CNN reported Friday night that the FBI interviewed him in 2011 at the request of a foreign government.
attended Bunker Hill Community College and wanted to become an engineer before he stopped school in 2008. As a young adult, he was a talented boxer - one former coach called him the best in Boston.
A student photographer made Tamerlan the subject of a photo essay in 2009 and focused on his life as an immigrant.
"I don't have a single American friend," he was quoted as saying. "I don't understand them."
Tamerlan was married and had a young daughter.
Both brothers were active on social media. Tamerlan had a YouTube account and created a playlist called "Terrorists."
Dzhokhar, 19, was a first-year biology student at the University of Massachusetts Dartmouth and won a $2,500 college scholarship from the city of Cambridge. He was an all-star wrestler at Cambridge Rindge and Latin School, where both boys attended, and was named Student Athlete of the Month as a senior.
But the transition to college, it seems, proved difficult for Dzhokhar, and he was failing most of his courses, according to the New York Times.
"Jahar," as his friends called him, was prolific on Twitter and has sent a few messages since the bombings. His last, on Wednesday, was a retweet of a Zimbabwean Muslim scholar: "Attitude can take away your beauty no matter how good looking you are or it could enhance your beauty, making you adorable." On Tuesday, a day after the bombing, he tweeted, "I'm a stress-free kind of guy."
The Tsarnaev family left their native Chechnya, a territory of Russia with a strong separatist movement, during a period of war in the early 1990s. Dzhokhar was apparently born in another country after the family fled.
Their father, an auto mechanic when he lived in America, has moved back to Russia.
The brothers are said to have become more interested in their Chechen roots in recent years.
On Twitter: @SeanWalshDN