LONDON

ED HUSAIN remembers the man as a kindly soul, not the sort you would suspect of recruiting for a radical Islamic group. As a teenager already in rebellion against his upstanding middle-class parents, who had raised him as a sort of Muslim choirboy, young Mohamed — his original first name — was an easy target.

They met in the early 1990s at the elaborate Muslim wedding of a distant relative. “He was a medic at Royal London Hospital, and he invited me to lunch,” said Mr. Husain, whose recently published memoir, “The Islamist: Why I Joined Radical Islam in Britain, What I Saw Inside and Why I Left,” has caused a ruckus in the newspapers, on television, on talks shows and in blogs.

“He was asking me questions and then saying, ‘White Muslims are being killed in Bosnia,’ ” he recalled in an interview. “What chances do we have as brown people in England?’ He was creating doubts.” He said his new friend had “black and white” answers to the world’s problems, and gave him books by Taqiuddin an-Nabhani, a Palestinian judge who, dissatisfied with the Muslim Brotherhood in the early 1950s, set up his own Islamic party, called Hizb ut-Tahrir, or Party of Liberation.

Thus began Mr. Husain’s journey into the world of British Islamic radicalism. He joined a university campus branch of Hizb ut-Tahrir. He said he had been hooked on an ideology that calls for a caliphate in Muslim countries and the end of Israel, though in nonviolent ways. Membership made him feel important, even though he was only a cog in a larger movement. “You feel a few cuts above an ordinary Muslim,” he said.