LONDON — He was just 12, the son of a former Conservative Party organizer in a neat suburban neighborhood of single-family homes and duplexes, when the father he adored died.

He started drinking, neighbors say. Getting in fights. But six months ago, Don Stewart-Whyte stopped drinking and smoking, and became calmer and more polite, those who know him say.

The 21-year-old had converted to Islam, the currency of some of the toughest and hippest young Asian students in his High Wycombe neighborhood.

“Islam answered all his questions, so he became a Muslim,” an Islamic habitue of the neighborhood west of London, Abid Zaman, said.

Today, Mr. Stewart-Whyte is being held with 22 other suspects in an alleged plot to blow up American-bound airliners over the Atlantic Ocean.

Mr. Stewart-Whyte, who became Abdul Waheed, and two other suspects in the plot were converts to Islam, reinforcing what many security experts and clerics had already said: The fervor and inexperience of new converts provides fertile soil for the allure of radical theology.

“The converts are seen as the most extreme, and they’re seen as the most extreme even by other Muslims who may not come from the U.K. Which is really worrying,” the director of the Brunel University Center for Intelligence and Security Studies in West London, Anthony Glees, said.

The growing number of home-grown converts in the ranks of militant Islam in Britain is raising troubling new questions not only about what it means to be British, but whether new Muslims must choose between family and faith across what many see as a yawning divide between civilizations.

Britain has perhaps 50,000 Islamic converts, ranging from fair-haired housewives in Yorkshire who have adopted the hijab to former Catholic priests, Afro-Caribbean street gang members and upper-middle-class university students.

At Islamic meetings attended by many new converts, Mr. Glees said, “people are brainwashed with certain ideas. Such as, there was no Holocaust. Such as, the London [transport] bombers killed far fewer people than the number of Muslims killed over hundreds of years by the British. These things are said, and they become increasingly accepted by these people as their ideological currency.”

“Of course, we have noticed this,” Abdurahman Anderson, who has worked extensively with new Muslims at south London’s Brixton Mosque, said. The congregation there, about 60% of them converts, has included Richard Reid, the British-born “shoe bomber” who was himself a convert, and September 11, 2001, co-plotter Zacarias Moussaoui, now serving a life sentence in America.

“A lot of youth …have had a kind of intellectual revolution,” Mr. Anderson said. “And with the world events, they’ve decided to get a fervor in themselves. We call it ‘hamas.’ This excitement can come to a new convert, or someone who’s turning away from the old, traditional Islam.

“What we find is that extremists have used this enthusiasm to try and teach them their erroneous ideas. And these individuals, who have a quest for knowledge, and an excitement, they’re susceptible to it.”

Friends say that after his conversion, Mr. Stewart-Whyte grew a beard, wore baggy trousers or the long “shalwar kameez” Islamic dress, and frequented a local Islamic studies center with two other young Muslims also arrested in the plot.

Only a few weeks before the arrests, he married a Moroccan woman who had moved into the house that he shared with his mother, a physical-education teacher. Neighbors said the young bride never emerged from the house without a full black burka, leaving only slits for her eyes. Neighbor Zaman, who claims he worked for a year as a driver for radical Islamic cleric Abu Hamza, defended the young convert.

“Don’s a nice guy,” he said. “He never talks about jihad. Just basic Islamic principles, love your neighbor and all that. You know, ‘Hi brother, how you doing?’ Nothing to do with terrorism.

“Of course, he was upset, like everybody is. You’ve got the U.S. selling these bunker-busting bombs to Israel, and they use those weapons to kill Lebanese men, women, and children — this is state-sponsored terrorism, you know what I mean?” he said. “But Don and all these Muslims that are in Britain, they’re working, they’ve got their wives, they’ve got their families.”

The purported plot to blow up liquid explosives on board aircraft “just doesn’t make sense to any of us,” he added. “Okay, a hole blows in the fuselage, and the plane starts going down, and you’re there with the rest of them, you’re bloody yelling and dying for five minutes? It’s crazy! Who would do that?”

Another High Wycombe convert arrested was Brian Young, 28, a former Rastafarian who became Umar Islam when he became a Muslim about three years ago. Young, who married to a young Islamic woman and is a recent father, worked as a city bus inspector. The Sun reported he was on duty the day of the London transport explosions in July 2005 and searched buses for other possible bombs. Accountant Oliver Savant, 25, was also a convert who lived with his Islamic wife, six months’ pregnant, in the east London area of Walthamstow. Neighbors said he was the son of an Iranian-born architect, while his mother, an accountant, was British by birth.

“He was the younger of two brothers. The older brother was a high flier in the city,” a neighbor, Hazel Kleinman, said, referring to London’s financial district.

Young and Savant were charged Monday with conspiracy to commit murder and preparing acts of terrorism. Mr. Stewart-Whyte was one of 11 suspects still under detention pending completion of the investigation.

With the large growth in Islamic converts, the number who have been drawn to violent Islam is statistically small.

Much more important, an Islamic convert and lecturer in Islamic studies at Cambridge University, Timothy Winter, said, is the potential for Western converts to inject new intellectual blood into the faith, not only expanding the reach of Islam but transforming it.

Yet the act of straddling a cultural divide inevitably raises the potential of a values gap, particularly wrenching for converts who have a foot in both camps.