When the term 'terrorism' is used in America, it usually refers to al-Qaeda or a related movement. However, the most deadly terror attack on US soil prior to 9/11 was the Oklahoma City Bombing, carried out by a right-wing white supremacists.

SPLCenter.org: Hate Groups Map

The Southern Poverty Law Center is one of the most prolific compilers of information on 'hate groups'. These tend to advocate violence of one form or another. The ones you've probably heard of include the Nation of Islam, the Ku Klux Klan and the Westboro Baptist Church.

So:

-Do you think domestic terrorism is a serious threat?
-Use the map and locate the hate group closest to you. Did you know they existed? Are you afraid?