I don't know how the government can have it both ways. They have big news conferences whenever they arrest a "terror cell", mostly nuts who rather like the notion of operating within the US by joining, however distantly, the Muslim terrorists. Witness Padilla, the shoe bomber, and that group out in Oregon.
Then when we ponder that such as a killer sniper on the lose as being possibly such a kook, we are tin foil suspects. I definitely think this shooter could very real be one of those kooks on a freelance terrorist job. I'm not yet convinced it's any nationwide terrorist attack or part of any organized mission of the Al Queda yet. But to speculate that this could be an nutty individual besmirched with the notion of being part of a great Jhiad is NOT out of the question.
The DC locale makes me very suspicious, however. It's also NOT out of the question that this could be the beginning of something much bigger.