16 April 2010
With respect to the threat posed by domestic terrorists and lone gunmen, I think the threat to national security comes more from the social reaction and policy response to attacks than from the attacks themselves (as with foreign terrorist attacks).
As far as capabilities go, obviously lone gunmen generally don't pose the same sort of large-scale threat as more organized, networked groups. The difficulty that they present is that there is no conspiracy and thus very limited opportunities for law enforcement to sniff out their plans. They are also pretty much impossible to prevent once in motion without establishing the trappings of a police state.
I don't think the threat capability of domestic terror groups comes as much from their attracting a large following and posing a direct threat to the government as much as their ability to pull off the same kind of attacks that foreign groups pull off. Let's not forget that, prior to '01, Oklahoma City was the gold standard for terror attacks on American soil. Further, there are indications from entities like the DHS and groups like the SPLC that these groups are currently on the uptick.