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Mohammad Atta and the 18 hijackers were strangers to the US. The Tsarnaev brothers weren't. They grew up in this country and had many friends and relatives here.
Dzhokhar Tsarnaev, 19, the younger brother, was an American citizen, read the oath of allegiance on September 11, 2012, on the anniversary of a day that changed the US.
Tamarlan Tsarnaev, though older at 26, was denied the honour because of he broke the law by hitting his girlfriend, according to his father Anzor Tsarnaev.
Did that turn him against the country? The father, as any parent, was indulgently dismissive. In interviews from Russia, where he lives, Anzor said his elder kept his faith.
"'Dad, I don't have to go anywhere!'" Anzor quoted his son in an interview to the New York Times.
"He works, his wife works, he has a child, they will give it to him! He didn't want to come (here). He had plans of his own. How could he leave everything and go?"
Tamarlan was a promising boxer once. And after winning his first regional fight he told a community paper in 2004 "I like the USA". But there were issues too.
He was lonely. "I don't have a single American friend," Tamarlan said during a photo shoot for a personal portfolio as a boxer. "I don't understand them."
He was studying accounting as a part-time student at Bunker Hill Community College in Boston. But his father said the young man also had other plans.
On a recent visit to Russia, Tamarlan told his father he could use his familiarity with the English language to work as a translator in Russia. And then, eventually, look for opportunities in China.
He had turned to religion five years ago, Zubeidat Tsarnaeva, his mother, told Russia Today. But he showed no jihadi inclinations - "he never told me he was on the side of jihad".
But early Friday morning, Tamarlan rushed headlong towards a group of police officers with explosives strapped to his body in a routinely jihadi fashion, cut down by police bullets.
As he fell to the ground his younger brother, Dzhokar Tsarnaev, drove their stolen Mercedes SUV over him. The 19-year-old was soon the country's most wanted.
Friends and their parents described him in various interviews as a funny and sociable individual. His parents, of course, thought he was an angel who could harm no one.
A classmate spoke of him as a genial friend who liked to wrestle despite a modest built - unlike his taller, more muscular brother, whom he was known to trail as a pup.
"He (Dzhokhar) came to the prom party we had," tweeted Robin Young, a radio host whose nephew and Dzhokahar were best friends. "Beautiful boy."
"Heart is broken. just confirmed, I know Dzhokhar Tsarnave, one of best freinds (sic) of my beloved nephew,who says he never in his life saw this," she tweeted further.
Dzhokar's classmates called TV news networks to talk about a the "angelic", "funny" and "sociable boy" , who, they swore, could never harm anyone.
Dzhokhar and his brother Tamarlan blew up two pressure cooker bombs at the finish line of the iconic Boston marathon killing three people, including an eight year old boy, they didn't know.
Then they killed - executed, more aptly - a policeman sitting in in his cruiser at MIT, and hurled grenades and homemade pipe- bombs at pursuing policemen three nights later.
Born to Chechen parents, they spent hardly any time in the troubled region, which has waged a violent insurrection against Russia since the disintegration of the Soviet Union.
Around 40 Chechen rebels took over a Moscow theatre in 2002 demanding the withdrawal of Russian forces from Chechnya. All them died, with 160 of their hostages in a Russian raid.
Two years later forces loyal to a Chechen commander took 1,100 people hostages including 777 children in a school in Russian federation's north caucasus region demanding independence of Chechnya. The Russian reaction was again swift and brutal, causing the death of 334 hostages including 186 children.
Chechens are also known to have contributed fighters to the ranks of al Qaeda to fight US-led international forces in Afghanistan.
But Anzor told his interviewers he tried to protect his children from their violent heritage. The family escaped to Kyrgyzstan, where they stayed till they relocated to the US in 2002.
Anzor returned to the region because of an illness. And the family, and its extended wings began breaking up. They were soon not talking to each other.
The sisters told authorities they had not been touch with their brothers Tamarlan and Dzhokhar for years.
And an uncle, Ruslan Tsarni, who lives in Maryland state and is estranged from the Tsarnaevs, believes the two brothers were losers. "They had failed to settle down themselves and hated anyone else who had."
He railed against them saying they had shamed the family, and their ethnic group by attacking Americans. He publicly urged Dzhokhar to surrender.
The young probably didn't hear him in his hideout in a trailered boat. But he did surrender hours later.