The Brothers Tsarnaev

The terrorist suspects next door.

Events in Boston were moving so quickly on Friday that it's impossible to draw too many conclusions. But the emergence of Dzhokhar and Tamerlan Tsarnaev as the chief terror suspects who paralyzed a great American city deserves at least some reflection.

One consoling thought is the admirable behavior of the citizens of greater Boston and its law enforcers. The point may seem banal, but it's no small matter that the public largely heeded the government's orders to stay off the streets and take the day off so police could track down the younger brother, 19-year-old Dzhokhar, who was captured Friday night after a day-long manhunt.

Bostonians have endured enormous disruption this week, but the city has shown a remarkable civility and calm throughout it all. Many lives were saved because of the rapid triage work by volunteers at the bomb scene. Bloomberg News reports that one of the marathon bombing's victims also helped the FBI identify a suspect after he awoke from surgery at the hospital. The suspect had dropped a bag at Jeff Bauman's feet and looked him in the eye minutes before it exploded. Mr. Bauman lost both legs below the knee but got his man.

As for the brothers, we will learn more about their motives, their training and whether they acted alone or as part of a network. What we have already learned is that they are immigrants from Chechnya, of the Muslim faith, and that 26-year old Tamerlan was uncomfortable in American society despite having lived here for about a decade.

The Associated Press reported that he was quoted in a Boston University student magazine in 2010 as saying, "I don't have a single American friend. I don't understand them." Mother Jones reported that a video attributed to a Tamerlan Tsarnaev extolled an extremist religious prophecy associated with al Qaeda. None of this is definitive but it might be illustrative.

alienation turned to jihad, it would not be the first time. The radicalization of young Muslims in the West, in particular children of the well-off, is by now a familiar story. The London bombers of 2005 were middle-class Pakistani immigrants from Birmingham. Faisal Shahzad, the failed Times Square bomber, was a naturalized citizen from Pakistan.

After the London bombings, many Americans took comfort in the belief that immigrants to the U.S. are better assimilated than they are in Europe. But that may be more conceit than fact, at least in regard to some young men. "My Son the Fanatic" is a novella by Hanif Kureishi that speaks to the difficulties of acculturation of second-generation Muslims. The recent Pulitzer Prize-winning play, "Disgraced," covers related ground.

Mitchell Silber and Arvin Bhatt explained how this can evolve into a threat in an instructive paper for the New York Police Department in 2007, "Radicalization in the West: The Homegrown Threat." The intelligence analysts looked at several cases here and abroad and described the process by which otherwise "unremarkable" men leading regular lives become jihadists.

"Muslims in the U.S. are more resistant, but not immune to the radical message," they wrote. "Despite the economic opportunities in the United States, the powerful gravitational pull of individuals' religious roots and identity sometimes supersedes the assimilating nature of American society which includes pursuit of a professional career, financial stability and material comforts." The Tsarnaev brothers may be an example.

Some will use this threat as an argument against immigration, but that would punish everyone for the sins of a few. The "homegrown" radical threat is really an argument for vigilance, especially within communities prone to producing terrorists.

This means surveilling foreign student groups in the U.S., certain immigrant communities that have produced jihadists, and, yes, even mosques and other Muslim venues. The key is to be familiar enough with these communities, to know and be trusted enough by their leaders, so those man and women will alert law enforcers when someone appears to have become radicalized.

This offends some civil libertarians, and the Associated Press excoriated the NYPD for the practice in a series of stories in 2011. In the wake of Boston, this looks notably misguided. New York's police say they've kept at it, under appropriate legal safeguards, and we hope they will continue.

The U.S. government watches right-wing extremist groups because we know they are dangerous. The police shouldn't refrain from doing the same to Muslim or immigrant groups merely because that is deemed less politically correct. As the week's events in Boston show, the costs of doing otherwise are too high.

A version of this article appeared April 19, 2013, on page A14 in the U.S. edition of The Wall Street Journal, with the headline: The Brothers Tsarnaev.