The federal government played a significant role in stopping the Ku Klux Klan and other white supremacist groups, especially during the civil rights movement. While state governments, police, and vigilante groups interacted with these groups in various ways, federal legislation and enforcement actions, such as the Civil Rights Act and the Voting Rights Act, helped to curb the power of the Klan and promote civil rights.
So, the most accurate response would be: the federal government.