An experimental procedure carried out by doctors at Mexico’s National Neurology and Neurosurgery Institute on some 500 unwitting patients caused 370 deaths between 1994 and 2007, activists said.

Representatives of groups like Democratic Doctors and Victims of Crimes Against Humanity, among others, told a press conference in Mexico City that the procedure was practiced without patients’ consent.

The organizations said that a complaint has now been filed with the Mexican Attorney General’s Office.