The wave of arrests and thwarted plots recently seen in the US has severely undermined the long-held assumption that American Muslims, unlike their European counterparts, are virtually immune to radicalisation. In reality, as argued in this ARI, evidence also existed before the autumn of 2009, highlighting how radicalisation affected some small segments of the American Muslim population exactly like it affects some fringe pockets of the Muslim population of each European country. After putting forth this argument, this paper analyses the five concurring reasons traditionally used to explain the divergence between the levels of radicalisation in Europe and the US: better economic conditions, lack of urban ghettoes, lower presence of recruiting networks, different demographics and a more inclusive sense of citizenship. While all these characteristics still hold true, they no longer represent a guarantee, as other factors such as perception of discrimination and frustration at US foreign policies could lead to radicalisation. Finally, the paper looks at the post-9/11 evolution of the homegrown terrorist threat to the US homeland and examines possible future scenarios.