The transformation of a quiet 19-year-old student on a college scholarship into one of America's most wanted terrorists continues to arouse the disbelief of people who knew him.

US security forces are still hunting down Dzhorkhar Tsarnaev, 19, who is believed to have placed the bombs that killed three people and injured more than 150 at the Boston Marathon earlier this week.

Tsarnaev came to the United States as a tourist from a Russian region near Chechnya in the early 2000s seeking asylum, official sources said.

His older brother Tamerlan, the 26-year-old who was killed in a shootout with police, reportedly joined him a few years later.

The pair lived together in Cambridge, outside Boston, where Dzhorkhar attended Cambridge Rindge and Latin School, graduating in 2011, according to his profile page on Russian social networking site Vkontakte.

"He was a very sweet kid in high school," Dzhorkhar's high schools class president recalls. "Normal; nothing weird about him.

"As far as I knew he wouldn't even hurt a fly. Everyone [who knows him] is in disbelief."

"He was normal," Lulu Emmons, who went to Rindge & Latin with Dzhokhar, told the Boston Globe. "He kind of fit in with everyone. Not really close with anyone, but he was friendly.

"I am just a little shocked. I sat next to this guy. I joked with him. I laughed with him. I had class with him. It is a little crazy," she said.

Dzhorkhar was awarded a $2,500 scholarship in May 2011 from the city of Cambridge, Massachusetts, to pursue higher education.

"Dzhokhar is a second-year medical student in the U.S. He is such an intelligent boy. We expected him to come on holidays here," Anzor Tsarnaev, who lives in the Russian city of Makhachkala, said.

Now Dzhokhar is on the run, described as "armed and dangerous" by authorities and accused of killing a police officer and carrying out a terrorist attack against the nation that gave him shelter more than a decade ago.

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