Answer:
The most likely reason that the Ku Klux Klan (KKK) is such an infamous extremist group in the United States is their history of extreme violence and intimidation. The KKK was originally formed in the aftermath of the American Civil War to oppose the civil rights of Black Americans and maintain white supremacy in the South. They used violent tactics such as lynchings, bombings, and arson to intimidate and terrorize Black Americans, as well as other minority groups and their allies. While the KKK is no longer as active or influential as it once was, their legacy of violence and hate has made them a notorious and infamous extremist group in US history. While they have had some political activity and influence, and have targeted other minority groups, their primary focus has always been on targeting Black Americans.