Wisconsin does have a "safe-place statute," Wis. Stat. section 101.11(1), which says that every employer is required to furnish both "safe employment" and a "safe place of employment." However, not much elaboration has been made on the length and extent employers must go to in order to meet this standard and employers do not have a general duty to protect their employees from criminal acts. Document all attempts to inform management of incidents like these and should you be suspended or terminated for complaining, then you would likely have a complaint to make under the National Labor Relations Act.
If you keep complaining and nothing is ever done and you don't feel safe, it may be worthwhile to retain counsel to communicate with the employer. Better to act than to wait for something bad to happen. Best of luck.