That is what is being challenged right now. Give us examples of extreme right-wing individuals in the US (as they claimed).
They are presumably referring to folks who are occasionally arrested for illegal possession of homemade explosives, although I suspect you get all kinds of people being nabbed for this, presumably after being ratted out by concerned neighbors after one too many window-rattling experiments in the back yard. The reference to "extreme right-wing" is gratuitous, given that all kinds of people experiment with them as a hobby.