A criminal behavior expert says the first attack in the Gangbuk District serial murder case appears to have functioned as a test of sedatives before later killings, pointing to a pattern of escalation rather than isolated acts. The suspect, a woman in her 20s identified by authorities as X, is accused of poisoning three men with drug-laced drinks, leaving two dead and one injured. She was handed over to prosecutors on Feb. 19 on charges including murder, causing injury and violating narcotics control laws. Oh Yun-seong, a professor of police administration at Soonchunhyang University, said in a radio interview Tuesday that the first incident involving the suspect’s boyfriend appears to have been an experiment to confirm the effects of benzodiazepine-based sleeping pills prescribed by a psychiatric clinic. “It was essentially testing the method,” he said, explaining that the suspect appeared to have observed that the drugged victim was unable to move for about four hours before moving on to more serious crimes. The boyfriend lost consciousness after drinking the beverage but later r