Gunmen killed 18 people at a clinic for drug addicts -- lining them up and shooting them one by one -- as a 24 hour surge in narcotics-related violence in Mexico claimed at least 40 lives. The bodies from the massacre were piled up in clinic at the troubled city of Ciudad Juarez, on the border with the United States, which has been ground zero in Mexico's growing drug war, state officials said. Four wounded were taken to hospitals around the city amid an alert for possible new gang attacks on the victims. In an eerily similar attack last year, eight people were gunned down at a Mexican clinic treating drug addicts.