In terms of terrorism in the US, that's not actually true by most counts. The US has seen more far-right terrorists than radical Islamic terrorists. Casualty counts for the Islamic terrorists are typically higher, but there are both more events and more individuals involved in far-right terrorism.

It's still worth noting that in terms of actual risk-per-individual both are astonishingly rare, and if your goal is to save lives that might otherwise be lost to violence there are far, far better ways of doing it.