I think it's usually true that these incidents are carried out by Muslims with no connections to "known" terrorists. The enemy we're fighting are Islamofascists who are schooled to believe that their mission in life is to kill "infidels". They don't need a plot to kill people in the ways they do. Just a car, or a bomb, or a gun, or whatever...
The enemy is the Islamofascist school of thought and the madrassas that teach it.
I think they all know that our gov't has learned how to foil large conspiracies (although some of our courts and Dem. politicians would like to stop that), so we'll be seeing more of these individual efforts to kill us, until we can find and shut down the madrassas and mosques who breed these people.
BTW, I do not believe that all Muslims want to kill us, but I do think that most of the people who do are from the extreme Muslim sects.