It appears at least 16 people have been killed and several wounded so far. Freakonomics author Professor Steven Levitt predicted such attacks more than a year ago. In his words :

I’d start by thinking about what really inspires fear. One thing that scares people is the thought that they could be a victim of an attack. With that in mind, I’d want to do something that everybody thinks might be directed at them, even if the individual probability of harm is very low. Humans tend to overestimate small probabilities, so the fear generated by an act of terrorism is greatly disproportionate to the actual risk.

and predicts what actually happened a few hours back

The basic idea is to arm 20 terrorists with rifles and cars, and arrange to have them begin shooting randomly at pre-set times all across the country. Big cities, little cities, suburbs, etc. Have them move around a lot. No one will know when and where the next attack will be. The chaos would be unbelievable, especially considering how few resources it would require of the terrorists.

In a follow up post he discusses about what can we do about it (emphasis mine):

But the obvious answer is a disappointing one: If terrorists want to engage in low-grade, low-tech terror, we are powerless to stop it.

That was an ominously prescient blog post from Professor Levitt.