The Tsarnaevs from top left to right: The father, Anzor; his brother, Ruslan Tsarni; Anzor's wife, Zubeidat; and their sons Tamerlan and Dzhokhar. In the background: the Boston bombing in April and a scene from Grozny in 1995. Photo Illustration by Gluekit; Reuters (Anzor Tsarnaev, Ruslan Tsarni, Boston bombing); Associated Press (Zubeidat Tsarnaev, Dzhokhar Tsarnaev); Agence France-Presse/Getty Images (Grozny); Getty Images (Tamerlan Tsarnaev)

When I first met Tamerlan Tsarnaev, now familiar as the elder of the two alleged Boston Marathon bombers, he gripped my hand like he was wringing out a rag. It was 2004, and Tamerlan had been in the U.S. for about a year, but he already had an outsize American dream. He planned to box for the U.S. Olympic Team one day, and he wanted to earn a degree, perhaps at Harvard or MIT, and to hold a full-time job at the same time, so he could buy a house and a car. I suggested he forget the house and the car during college, as most American students do. He didn't see why he should.
I was on sabbatical that year, taking classes at Harvard on a journalism fellowship, and had wanted to meet some of the refugees from Russia's war to reconquer the breakaway Muslim region of Chechnya. I expected to write about Russia's Islamist insurgency in the future, and I thought some Chechen expatriates might help me with my stories.
A friend told me that his mother had rented an apartment to some Chechens. He drove me to a weather-beaten three-family home crammed between others in a tattered corner of Cambridge, Mass. I was led up a narrow stairway, littered with shoes and slippers, to their third-floor apartment—the start of a relationship that came full circle last April, when I encountered the Tsarnaevs again under very different circumstances.

Dzhokhar Tsarnaev emerges from a boat on April 19 in Watertown, Mass., at the time of his capture. The red dot of a rifle laser sight is on his forehead. Massachusetts State Police/Associated Press
A decade ago, there was nothing about the Tsarnaevs to suggest any involvement in Islamist extremism. But they already seemed like "losers," as their successful Americanized uncle told reporters after the attack. They were out of place in the U.S., and my relationship with them developed because they needed so much basic advice about how to get by. I didn't sense impending danger in their household, but looking back, I can see now that I glimpsed a new type of threat to the U.S., one that we have only recently begun to confront.
The father, Anzor, was a lean man with a square jaw who seldom smiled. The mother, Zubeidat, was a wide-eyed rapid talker with a low-cut dress and high heels who waved her arms and teased her black hair like the pop singer Cyndi Lauper. Their two daughters and younger son, Dzhokhar, then 11, greeted me and retreated to a corner of the room.........