I asked myself: what is the most lethal terrorist group in the history of the US? I poked around for some numbers and facts. Here's my verdict:
The Ku Klux Klan and affiliates are known to have lynched thousands of African Americans since their founding in 1865. The numbers I found: ~3500 total black lynchings to date directly attributable to the Klan, or ~120 per year, although the Klan has had several periods of growth and then collapse. Note that a large number of whites were also lynched. There are, of course, various Klan side organizations and freelance racists, so it's hard to get firm numbers.
The Klan ran local politics in several States for many decades and directly intimidated an uncountable number of people across the country...for generations. Most young Americans don't realize just how openly the KKK used to weigh on American politics. This should give you an idea:
Yes, they marched that openly in Washington D.C. in 1928, and have held many large, public gatherings since.
Of course, the Klan is best known for advocating segregation and fighting against equality, whilst packaging their very anti-Christian arguments in faux Christian symbology e.g. crosses. Through their actions, the Klan also spawned hateful counter-groups which in turn also committed hateful acts; hatred, of course, begets hatred.
Let's remember, as we speak about terrorism from abroad, that the single worst terrorist group in the history of our nation was born and bred right here, and still exists openly in many States. And whether that terrorizes you probably has a lot to do with your skin color.