There are a lot of terrorist attacks that are scary. We have lots to pick from. About a week ago, we were reminded of the threat of a dirty bomb exploding in our cities when a man was arrested for smuggling 80 grams of 90% enriched radioactive material. Then, they could load up tractor trailers with fertilizer and run them into buildings, like the guy who couldn't speak english well and had no interest in learning how to drive the truck backwards evidently wanted to do.
But the thing that scares me most is a coordinated campaign of these. It's extremely easy to pull off, and extremely hard to prevent without restricting civil liberties to a level that I refuse to go to. I'm just glad that this one seems to be a solo act, and that an off duty cop was able to respond quickly enough to keep even more people from dying.