Last year, a likable and accomplished scientist named Serguei Popov, who for nearly two decades developed genetically engineered biological weapons for the Soviet Union, crossed the Potomac River to speak at a conference on bioterrorism in Washington, DC. Popov, now a professor at the National Center for Biodefense and Infectious Diseases at George Mason University, is tallish, with peaked eyebrows and Slavic cheekbones, and, at 55, has hair somewhere between sandy and faded ginger. He has an open, lucid gaze, and he is courteously soft-spoken. His career has been unusual by any standards. As a student in his native city of Novosibirsk, Siberia’s capital, preparing his thesis on DNA synthesis, he read the latest English-language publications on the new molecular biology. After submitting his doctorate in 1976, he joined Biopreparat, the Soviet pharmaceutical agency that secretly developed biological weapons. There, he rose to become a department head in a comprehensive program to genetically engineer biological weapons. When the program was founded in the 1970s, its goal was to enhance classical agents of biological warfare for heightened pathogenicity and resistance to antibiotics; by the 1980s, it was creating new species of designer pathogens that would induce entirely novel symptoms in their victims.