Researchers are investigating a long unknown killing field in Cambodia with the graves of thousands of Khmer Rouge victims from the 1970s. But local villagers found it first. By the time the researchers arrived last week, some 200 graves had been dug open and the bones scattered through the woods by hundreds of people hunting for jewelry. After the assault on the burial ground, the village seemed filled with remorse and dread. Several people said they had been awakened at night by screams from the graves.