JUST hours after the two suspects in the Boston Marathon bombing were identified as Muslims, Representative Peter T. King of New York, the Republican chairman of the House Subcommittee on Counterterrorism and Intelligence, called for an “increased surveillance” of Islamic communities in the United States. “I think we need more police and more surveillance in the communities where the threat is coming from,” he told National Review. “The new threat is definitely from within.”

Mr. King’s hypothesis, and the widespread surveillance policies already in effect since 9/11, assume that the threat of radicalization has become a matter of local geography, that American Muslims are creating extremists in our mosques and community centers.

But what we’re learning of the suspects, the brothers Tamerlan and Dzhokhar Tsarnaev, suggests a different story, and one that has itself become familiar: radicalization does not happen to young people with a strong grounding in the American Muslim mainstream; increasingly, it happens online, and sometimes abroad, among the isolated and disaffected.

The YouTube page of Tamerlan Tsarnaev, for example, does not contain a single lecture from a scholar, imam or institution in America. One report suggests that he found the theology taught in a local Cambridge mosque, the Islamic Society of Boston, unpalatable: while attending a Friday service in which an imam praised the life and work of the Rev. Dr. Martin Luther King Jr., Mr. Tsarnaev shouted that the imam was a “nonbeliever.” The younger Tsarnaev brother seems to have rarely attended a mosque at all.