Well I can't define a McVeigh extremist. If you mean anti-government white extremists, I'd say a lot. Remember The Whethermen (or Whether Underground?) How about all the Christian militias. Before 9/11 every act of terrorism committed on American soil was committed by an AMerican.

As for Jewish extremists, take a walk through the middle-east, the violence is far from one-sided. When radical Jews aren't happy with the peace process, they attack mosques (Hebron '94), marketplaces (Hebron again) and even assasinate Israeli leaders. Sharon massacres refugees. Leaders bulldoze homes. Kahane (an elected member of the Israeli government tried to pass a law that would make it illegal for Jews to marry Arabs). Sounds like a lot of extremists to me.