How does the internet affect the radicalisation of extreme-right lone actor terrorists? In the absence of an established theoretical model, this article identifies six mechanisms seen as particularly relevant for explaining online radicalisation. Having first reviewed a larger set of relevant lone actor terrorists, the study traces these mechanisms in three selected cases where the internet was reportedly used extensively during radicalisation. The findings show that the internet primarily facilitated radicalisation through information provision, as well as amplifying group polarisation and legitimising extreme ideology and violence through echoing. In all three cases, radicalisation was also affected considerably by offline push-factors that through their presence made extreme online messages more impactful. The results challenge the view that offline interaction is necessary for radicalisation to occur but also the view that online influence itself is sufficient.