AMERSFOORT, The Netherlands -- Jason Walters was the consummate outsider, his neighbors recalled, the loner who never quite fit in, the one the other kids liked to bully at school. He was the son of a black American father and a Dutch woman, and had few friends. He was pro-American, they said, perhaps because of his father. Suddenly last month, Walters, 19, was arrested after a violent hours-long standoff with police, in one of a series of raids on suspected Muslim terrorist cells following the Nov. 2 assassination of filmmaker Theo van Gogh by an alleged Muslim extremist. According to...