It's hard to comment too much at this early stage, but these sort of incidents are getting all too common over there.
We, me included, often have a moan about our firearms licencing, which is far from perfect, but the point of it is to allow people to have firearms when they can show a good reason to have them and are not a danger to others.
Neither of these are a consideration in the USA, where there are absolutely no checks if you buy a firearm privately. Is it any wonder they end up in the wrong hands? It's a long time since the USA was a frontier and the Indians tend not to want to shoot arrows in to you and take your scalp so much these days. It is about time that people who exercise their right to have firearms also should have to show they are up to the responsibility that ownership should entail.