I have spent the better part of today listening to interviews of people who specialized in security, all of whom say the same thing. Unless you think there is a massive press conspiracy, what reason would they all have to lie?
For example, it's actually MORE comforting believing a famous leader was assassinated by a massive government/international conspiracy than by a lone kook..strangely, the thought that a lone person or small group of fairly ordinary people can cause horrific death and destruction is worse than the idea that that massive governmental conspiracies could do that.
Also, people HATE randomness. If something bad happens, it's comforting that it was the result of a grand plan involving lots of people..makes it seem to have some sort of larger purpose, no matter how evil.
But the problem is it's quite possible for a few intelligent people to engage in massive destruction without the support of thousands in shadowy agencies or foreign governments.