[syriacus]

Could Al-Qaeda be behind it, right here in the USA, trying to achieve by terror what they have failed to accomplish on the battlefield? More asymmetric warfare?

Could be some "motivated" individuals like the DC Snipers, and the mall shooters, and the men who have driven SUVs into crowds of people.

[syriacus]

The Rise of Low-Tech Terrorism

But it's a mistake to write off the angry amateurs. They're not terribly skilled, but it doesn't take that much skill to kill dozens of people