It was about a year ago that neighbors saw a change in Osman Khatib, a transformation far more profound than the usual whimsy that can take hold of a young man in his late teens. For as long as anyone can remember, Mr. Khatib had dressed like most British teenagers, favoring jeans, T-shirts, sneakers and other Western-style fashion. But last year, he abruptly adopted a new look that apparently reflected a deeper sense of devotion on the part of the British-born Muslim from East London. Full Story