The most dangerous terrorist organization to Americans

Which terrorist organization do you think is the most dangerous to Americans and American interests? Instead of an organization, you may think a lone wolf attacker is the most dangerous. Whichever you decide, explain why you selected that organization or individual. Think about their history, the impact of their attacks, their weapons, and their capabilities (such as if they have a long-range reach or have access to a large amount of money). Also, provide a short summary of the group or individual.