"Behind closed doors" or "crowdless" matches in the major North American sports leagues are exceedingly rare. Local law enforcement, private security contractors working for either the team or a league, and national agencies such as the United States Department of Homeland Security take large roles in preventing situations of fan violence before they can occur by restricting access to known troublesome fans either at the gate or even at the stage of selling tickets (such as "do not sell" lists), along with heavy restrictions on bringing in items and screening with metal detectors and pat-down searches where bringing in a weapon or explosive device can result in immediate arrest and lifetime ejection from a venue, and other examples such as the "clear bag" policy, which only allows spectators to bring in bags that can be easily seen through.