725 ILCS 205/3

When any person is charged with a criminal offense and it shall appear to the Attorney General or to the State's Attorney of the county wherein such person is so charged, that such person is a sexually dangerous person, within the meaning of this Act, then the Attorney General or State's Attorney of such county may file with the clerk of the court in the same proceeding wherein such person stands charged with criminal offense, a petition in writing setting forth facts tending to show that the person named is a sexually dangerous person.