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.

Representative King’s theories also fail to explain why, if young people are being radicalized within mainstream Islamic communities, there aren’t more attacks like the one in Boston. By some measures Islam is the fastest-growing religion in the United States, and the last decade has seen a rapid expansion of Muslim institutions across the country.

Yet what’s most obvious to anyone who has spent time in these communities is that whether they are devotional or educational, focused on the arts or on interfaith cooperation and activism, this mediating set of American Muslim institutions is keeping impressionable young Muslims from becoming radicalized.

Take the Islamic Society of Boston Cultural Center and its range of devotional, arts and educational programs, from preschool to a seminary. Or Chicago’s Inner-City Muslim Action Network, complete with a medical clinic, civic leadership education and a summer music festival that draws on the biggest names of Muslim hip-hop to promote peace through community organizing. Or Zaytuna College in Berkeley, Calif., the nation’s first four-year Islamic liberal arts school.

These institutions and others have different aims, but they abide by a common idea: if the center of Judaism is the law, and the heart of Christianity is love, what Islam requires, above all else, is mercy. And whether on display in health care provided for the poor at South Los Angeles’s UMMA Community Clinic, or in a patiently handled Arabic lesson that will one day lead a new convert into the fullness of the tradition, Islamic mercy, preached and practiced within the community, allows no room for radicalization.

Representative King and others have it exactly, completely wrong — the American Muslim community has actively and repeatedly, day in and day out, rejected such radicals on religious grounds: they do not know mercy.

More than a decade since 9/11, this should no longer be any secret. Across the nation, the doors are open, and more are opening every day. And despite whatever misplaced fears the Boston bombings evoke about radical Islam and homegrown terror, we’ll all find ourselves increasingly secure as more Muslims heed the call — coming to Islam as it is in the United States, as a real, living community.

Suhaib Webb is the imam of the Islamic Society of Boston Cultural Center. Scott Korb, who teaches writing at New York University and the New School, is the author of “Light Without Fire: The Making of America’s First Muslim College.”

A version of this op-ed appeared in print on April 25, 2013, on page A31 of the New York edition with the headline: No Room for Radicals.