[pnako]

The last few cases that come to mind (Breivik, Tarrant) are from lone wolves that had radical ideas but were not affiliated with any group.

It must be very difficult to catch (unless you want a surveillance society where everyone is suspect).

Even in cases where it's planned and announced it's difficult to stop (Al Qaeda did announce they would assassinate Charb, editor in chief of Charlie Hebdo, and despite police protection they managed to do so).