The C.I.A.’s concerns were not unwarranted. In 2019, the Russians undertook an elaborate operation to find Mr. Poteyev, forcing a scientist from Oaxaca, Mexico, to help. The scientist, Hector Alejandro Cabrera Fuentes, was an unlikely spy. He studied microbiology in Kazan, Russia, and later earned a doctorate in the subject from the University of Giessen in Germany. He was a source of pride for his family, with a history of charitable work and no criminal past. But the Russians used Mr. Fuentes’s partner as leverage. He had two wives: a Russian living in Germany and another in Mexico. In 2019,...