This is from Peter Bergen of the New America Foundation, a top expert, writing at CNN.com:
Of the 380 extremists indicted for acts of political violence or for conspiring to carry out such attacks in the U.S. since 9/11, 81 were able to obtain explosives or the components necessary to build a bomb, according to a count by the New America Foundation.
Of those, 51 were right-wing extremists, 23 were militants inspired by al Qaeda's ideology, five have been described as anarchists and one was an environmentalist terrorist.
But in the years since 9/11, actual terrorist bombings in the U.S., like the ones at the Boston Marathon, have been exceedingly rare.
The only bombing attack carried out by an extremist in the United States during the past 12 years was in 2004 when Dennis Mahon, a white supremacist, sent a homemade bomb to Don Logan, the African-American city diversity director of Scottsdale, Arizona, who was maimed when the package exploded in his arms.
Bergen then goes on to explain why there have been so few successfull attacks, and the reasons are pretty self-evident. Basically, intel agencies and cops started paying a hell of a lot more attention. Also, civic groups and regular people:
In January 2010, Kevin Harpham, a white supremacist, placed a backpack filled with explosives along the route of a Martin Luther King Jr. parade in Spokane, Washington. Fortunately, city workers spotted the suspicious package and police were able to defuse the bomb before the parade began.
Putting wholly to the side who is or isn't responsible for this, extremist right-wing terrorism is clearly a problem in this country, and conservatives should be among the quickest to denounce it. I reiterate here, however, my great hope that this was a nonpolitical loner. Would certainly be the "best" scenario.