From what I've read so far, it seems a rather crazy young man showed up with a 9mm Glock 19 equipped with a 30 round mag (amazingly the press actually reported it with correct nomenclature...) and opened up on the Congresswoman and her entourage at a public event, managing to kill a defenseless little girl along with the others. That last little bit there is what bothers me the most. No doubt this will, unfortunately, end up being used by the Anti's as a cry for a return to magazine restrictions.
I also noted earlier in the reports that a CCW carrier or possibly private security person may have returned fire, causing the shooter to attempt to flee. It sounds like he didn't make it very far before being stopped by bystanders until the police arrived. Lessons to be
Best of luck to the Congresswoman, and to the other wounded, and condolences to the families of those killed.