The first time they commit a crime they kill people or have destroyed valuable US assets such as military bases, damns, power plants and the like. Homeland Security needs to discover these terrorists before their first act of terrorism. Once they have proof they must be put on the Do Not Fly list with the option to appeal and they need to be unable to purchase any weapon. They need to stay on both lists until they win their appeal. Otherwise, they're on those lists for life.