I've seen various reports citing unnamed sources claiming group involvement and not just something "middle school kids" would do. Here's one example...
One key point here was pretty effectively debunked by another Freeper (maybe even earlier on this thread). These were not sophisticated bombs, according to this Freeper who claimed to be an electrical engineer. That reinforced something that I had suspected from the beginning. And now, nearly a week later, these bombs look like something a few of my middle-school classmates could have built -- even without any help from their parents, let alone a group of terrorists.