Later, friends and relatives would recall both as seemingly incapable of terrorism. The brothers were part of an ethnic Chechen family that came to the U.S, in 2002, after fleeing troubles in Kyrgyzstan and then Dagestan, a predominantly Muslim republic in Russia's North Caucasus. They settled in a working-class part of Cambridge, where the father, Anzor Tsarnaev, opened an auto shop.
Dzhokhar did well enough in his studies at prestigious Cambridge Rindge and Latin to merit a $2,500 city scholarship for college.
Tamerlan, though, could be argumentative and sullen. "I don't have a single American friend," he said in an interview for a photo essay on boxing. He was clearly the dominant of the two brothers, a former accounting student with a wife and daughter, who explained his decision to drop out of school by telling a relative, "I'm in God's business."
It's not that Tamerlan Tsarnaev didn't have options. For several years he'd impressed coaches and others as a particularly talented amateur boxer.
"He moved like a gazelle. He could punch like a mule," said Tom Lee, president of the South Boston Boxing Club, where Tsarnaev began training in 2010."I would describe him as a very ordinary person who didn't really stand out until you saw him fight."
But away from the gym, Tamerlan swaggered around his parents' home like he owned it, those who knew him said. And he began declaring an allegiance to Islam, joined with increasingly inflammatory views.
One of the brothers' neighbors, Albrecht Ammon, recalled an encounter in which the older brother argued with him about U.S. foreign policy, the wars in Afghanistan and Iraq, and religion. The Bible, Tamerlan told him, was a "cheap copy" of the Quran, used to justify wars with other countries. "He had nothing against the American people," Ammon said. "He had something against the American government."
Dzhokhar, on the other hand, was "real cool," Ammon said. "A chill guy."
Since the bombing, the younger brother had maintained much of that sense of cool, returning to classes at the University of Massachusetts-Dartmouth and attending student parties.
On the day of the bombing, he wrote on Twitter: "There are people that know the truth but stay silent & there are people that speak the truth but we don't hear them cuz they're the minority."
But by Tuesday, when he stopped by a Cambridge auto garage, the mechanic, accustomed to long talks with Dzhokhar about cars and soccer, noticed the normally relaxed 19-year-old was biting his nails and trembling.
The mechanic, Gilberto Junior, told Tsarnaev he hadn't had a chance to work on a car he'd dropped off for bumper work. "I don't care. I don't care. I need the car right now," Junior says Dzhokhar Tsarnaev told him.
Now, with the photos out, it was time to move. Already, one of Dzhokhar's college classmates had taken to studying the photo of Suspect No. 2 — nearly certain it was his friend, although others were skeptical. It wouldn't take long for others to notice.
The call to the police dispatcher came in at 10:20 p.m. Thursday: shots fired on the Massachusetts Institute of Technology campus in Cambridge. Ten minutes later, when police arrived to investigate, they found one of their own, university officer Sean Collier, shot multiple times inside his cruiser. He had been monitoring traffic near a campus entrance, said Cambridge Police Commissioner Robert Haas.
The baby-faced 26-year-old Collier, in just a year on patrol, had impressed both his supervisors and the students as particularly dedicated to his work. A few days earlier, he'd asked Chief John DiFava for approval to join the board at a homeless shelter, in a bid to steer people away from problems before they developed. Now he was being pronounced dead at the hospital.
Witnesses reported seeing two men. Fifteen minutes later, another call came in of an armed carjacking by two men. That was on Brighton Avenue, Haas said. For the next half-hour, the carjacking victim was kept in his car, had his bank card used to pocket $800 from an ATM and was told by his captors that they'd just killed a police officer and were responsible for the bombing, Watertown Police Chief Edward Deveau said. Haas said the man escaped from the car when his captors went into a Cambridge gas station, and he called police.
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